Refactor|Client: Moved alignment flags to a separate header, added templates
libdeng2|Rectangle: Added methods (area, isNull, moved)
Refactor|libgui: Interface cleanup
Client|Widgets: GuiRootWidget has a public uniform for the shared atlas

All widgets can use GuiRootWidget::uAtlas() in their shaders to bind
to the shared UI atlas.
Client|Widgets: Apply Alignment template, GLTextComposer color argument
Client|Widgets: Cleanup