Applications: Add a new Help app

This is a neat simple app that can display the Serenity manual ^)

Applications/Help/main.cpp

@@ -0,0 +1,79 @@
+#include "ManualModel.h"
+#include <LibCore/CFile.h>
+#include <LibDraw/PNGLoader.h>
+#include <LibGUI/GApplication.h>
+#include <LibGUI/GMessageBox.h>
+#include <LibGUI/GSplitter.h>
+#include <LibGUI/GTextEditor.h>
+#include <LibGUI/GTreeView.h>
+#include <LibGUI/GWindow.h>
+#include <LibHTML/HtmlView.h>
+#include <LibHTML/Layout/LayoutNode.h>
+#include <LibHTML/Parser/CSSParser.h>
+#include <LibHTML/Parser/HTMLParser.h>
+#include <LibMarkdown/MDDocument.h>
+
+int main(int argc, char* argv[])
+{
+    GApplication app(argc, argv);
+
+    auto window = GWindow::construct();
+    window->set_title("Help");
+    window->set_rect(300, 200, 570, 500);
+
+    auto splitter = GSplitter::construct(Orientation::Horizontal, nullptr);
+
+    auto model = ManualModel::create();
+
+    auto tree_view = GTreeView::construct(splitter);
+    tree_view->set_model(model);
+    tree_view->set_size_policy(SizePolicy::Fixed, SizePolicy::Fill);
+    tree_view->set_preferred_size(200, 500);
+
+    auto html_view = HtmlView::construct(splitter);
+
+    extern const char default_stylesheet_source[];
+    String css = default_stylesheet_source;
+    auto sheet = parse_css(css);
+
+    tree_view->on_selection_change = [&] {
+        String path = model->page_path(tree_view->selection().first());
+        if (path.is_null()) {
+            html_view->set_document(nullptr);
+            return;
+        }
+
+        dbg() << "Opening page at " << path;
+
+        auto file = CFile::construct();
+        file->set_filename(path);
+
+        if (!file->open(CIODevice::OpenMode::ReadOnly)) {
+            int saved_errno = errno;
+            GMessageBox::show(strerror(saved_errno), "Failed to open man page", GMessageBox::Type::Error, GMessageBox::InputType::OK, window);
+            return;
+        }
+        auto buffer = file->read_all();
+        StringView source { (char*)buffer.data(), buffer.size() };
+
+        MDDocument md_document;
+        bool success = md_document.parse(source);
+        ASSERT(success);
+
+        String html = md_document.render_to_html();
+        auto html_document = parse_html(html);
+        html_document->normalize();
+        html_document->add_sheet(sheet);
+        html_view->set_document(html_document);
+
+        String page_and_section = model->page_and_section(tree_view->selection().first());
+        window->set_title(String::format("Help: %s", page_and_section.characters()));
+    };
+
+    window->set_main_widget(splitter);
+    window->show();
+
+    window->set_icon(load_png("/res/icons/16x16/book.png"));
+
+    return app.exec();
+}