Client|Default Style: Added square logo graphic, tweaked fonts of OS X
libdeng2|Vector|Rectangle: More methods for Vector and Rectangle templates
libdeng2|RuleRectangle: Clearing an input rule
Client|Cleanup: GuiRootWidget provides a texture with a solid white pixel

A shared texture with a single white pixel is quite useful in many
widgets (at least until skin textures are added).
Client|Widgets: Added LabelWidget

A label consists of an image plus styled text (both are optional). This
will serve as a base class for the ButtonWidget.