First part of the theming system rewrite.
Better "common" view.
Added <include> tag.
Theme applicators have become the virtual method
GuiComponent::applyTheme(theme, view, element, properties).
Applying fonts works now.
Added support for theme "extras".
Fixed a few crashes (e.g. TextListComponent::applyTheme).
Changed the sound cache to be part of the Sound class instead of
ThemeData.
Rewrote the theming documentation.