LibWeb: Add a DOM Event class (instead of events being simple strings)

This patch adds the Event base class, along with a MouseEvent subclass.
We now dispatch MouseEvent objects for mousedown, mouseup and mousemove
and these objects have the .offsetX and .offsetY properties.

Both of those properties are hard-coded at the moment. This will be
fixed in the next patch. :^)

Libraries/LibWeb/Bindings/MouseEventWrapper.cpp

@@ -0,0 +1,67 @@
+/*
+ * Copyright (c) 2020, 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 <AK/Function.h>
+#include <LibJS/Runtime/Function.h>
+#include <LibWeb/Bindings/MouseEventWrapper.h>
+#include <LibWeb/DOM/MouseEvent.h>
+
+namespace Web {
+namespace Bindings {
+
+MouseEventWrapper::MouseEventWrapper(MouseEvent& event)
+    : EventWrapper(event)
+{
+    put_native_property(
+        "offsetX",
+        [this](JS::Object*) {
+            return JS::Value(this->event().offset_x());
+        },
+        nullptr);
+    put_native_property(
+        "offsetY",
+        [this](JS::Object*) {
+            return JS::Value(this->event().offset_y());
+        },
+        nullptr);
+}
+
+MouseEventWrapper::~MouseEventWrapper()
+{
+}
+
+const MouseEvent& MouseEventWrapper::event() const
+{
+    return static_cast<const MouseEvent&>(EventWrapper::event());
+}
+
+MouseEvent& MouseEventWrapper::event()
+{
+    return static_cast<MouseEvent&>(EventWrapper::event());
+}
+
+}
+}