LibWeb: Basic implementation of global event handlers :^)

Document and HTMLElement now inherit from HTML::GlobalEventHandlers
which allows them to support "onfoo" event handler attributes.

These are assignable both via IDL attributes and content attributes.

Event listeners constructed this way get a special "attribute" flag
on them so we know which one to replace if you reassign them.
This also allows them to coexist with EventTarget.addEventListener().

This is all a bit sloppy, but it works decently for a first cut.
The Window object should also inherit GlobalEventHandlers, but since
we don't generate it from IDL, I haven't taken that step here.

Also this would be a lot nicer if we supported IDL mixins.

Userland/Libraries/LibWeb/HTML/GlobalEventHandlers.cpp

@@ -0,0 +1,99 @@
+/*
+ * Copyright (c) 2021, Andreas Kling <kling@serenityos.org>
+ * All rights reserved.
+ *
+ * Redistribution and use in source and binary forms, with or without
+ * modification, are permitted provided that the following conditions are met:
+ *
+ * 1. Redistributions of source code must retain the above copyright notice, this
+ *    list of conditions and the following disclaimer.
+ *
+ * 2. Redistributions in binary form must reproduce the above copyright notice,
+ *    this list of conditions and the following disclaimer in the documentation
+ *    and/or other materials provided with the distribution.
+ *
+ * THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS "AS IS"
+ * AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
+ * IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE
+ * DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT HOLDER OR CONTRIBUTORS BE LIABLE
+ * FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
+ * DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR
+ * SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER
+ * CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY,
+ * OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
+ * OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
+ */
+
+#include <LibJS/Interpreter.h>
+#include <LibJS/Parser.h>
+#include <LibJS/Runtime/ScriptFunction.h>
+#include <LibWeb/DOM/Document.h>
+#include <LibWeb/DOM/EventListener.h>
+#include <LibWeb/HTML/EventHandler.h>
+#include <LibWeb/HTML/EventNames.h>
+#include <LibWeb/HTML/GlobalEventHandlers.h>
+#include <LibWeb/UIEvents/EventNames.h>
+
+namespace Web::HTML {
+
+#undef __ENUMERATE
+#define __ENUMERATE(attribute_name, event_name)                              \
+    void GlobalEventHandlers::set_##attribute_name(HTML::EventHandler value) \
+    {                                                                        \
+        set_event_handler_attribute(event_name, move(value));                \
+    }                                                                        \
+    HTML::EventHandler GlobalEventHandlers::attribute_name()                 \
+    {                                                                        \
+        return get_event_handler_attribute(event_name);                      \
+    }
+ENUMERATE_GLOBAL_EVENT_HANDLERS(__ENUMERATE)
+#undef __ENUMERATE
+
+GlobalEventHandlers::~GlobalEventHandlers()
+{
+}
+
+void GlobalEventHandlers::set_event_handler_attribute(const FlyString& name, HTML::EventHandler value)
+{
+    auto& self = global_event_handlers_to_event_target();
+
+    RefPtr<DOM::EventListener> listener;
+    if (!value.callback.is_null()) {
+        listener = adopt(*new DOM::EventListener(move(value.callback)));
+    } else {
+        StringBuilder builder;
+        builder.appendff("function {}(event) {{\n{}\n}}", name, value.string);
+        auto parser = JS::Parser(JS::Lexer(builder.string_view()));
+        auto program = parser.parse_function_node<JS::FunctionExpression>();
+        if (parser.has_errors()) {
+            dbgln("Failed to parse script in event handler attribute '{}'", name);
+            return;
+        }
+        auto* function = JS::ScriptFunction::create(self.script_execution_context()->interpreter().global_object(), name, program->body(), program->parameters(), program->function_length(), nullptr, false, false);
+        ASSERT(function);
+        listener = adopt(*new DOM::EventListener(JS::make_handle(static_cast<JS::Function*>(function))));
+    }
+    if (listener) {
+        for (auto& registered_listener : self.listeners()) {
+            if (registered_listener.event_name == name && registered_listener.listener->is_attribute()) {
+                self.remove_event_listener(name, registered_listener.listener);
+                break;
+            }
+        }
+        self.add_event_listener(name, listener.release_nonnull());
+    }
+}
+
+HTML::EventHandler GlobalEventHandlers::get_event_handler_attribute(const FlyString& name)
+{
+    auto& self = global_event_handlers_to_event_target();
+    for (auto& listener : self.listeners()) {
+        if (listener.event_name == name && listener.listener->is_attribute()) {
+            return HTML::EventHandler { JS::make_handle(&listener.listener->function()) };
+        }
+    }
+
+    return {};
+}
+
+}