ThemeEditor: Give each theme property its own editing widget

This is quite a radical change. The previous UI was very easy to add new
properties to (just add to the FooRole enum and it automatically
works), but not so nice to use: A ComboBox for selecting a property,
and then a box to edit that property's value. This makes it difficult
to compare different properties or edit multiple together without a lot
of back-and-forth.

This new design gives each property its own editing widgets, with those
categorized into several tabs. To try and avoid increasing the
maintenance burden for this, the UI is generated from the
`windows_tab`, `widgets_tab` and `syntax_highlighting_tab` variables,
which are basically just lists of which properties go in that tab. One
of the `FooProperty.gml` files is loaded to create each property's
widgets.

Userland/Applications/ThemeEditor/main.cpp

@@ -65,7 +65,7 @@ ErrorOr<int> serenity_main(Main::Arguments arguments)
         return main_widget->request_close();
     };
 
-    window->resize(480, 520);
+    window->resize(820, 520);
     window->set_resizable(false);
     window->show();
     window->set_icon(app_icon.bitmap_for_size(16));