qmake: Don't build libgui if client is disabled
Fixed|Client|LabelWidget: Drawing text at the correct position
Refactor|Client|LabelWidget: Allow subclasses to modify widget behavior

A subclass may augment the label's geometry or modify the MVP matrix.

Todo for later: Some of this may be generalizable to GuiWidget level
to allow a similar mechanism to be used with other widgets as well.
Merge branch 'master' of ssh://github.com/skyjake/Doomsday-Engine into ui-framework
Client|LogWidget: Fixed scrolling glitches, scroll to top/bottom
Client|GuiRootWidget|VertexBuilder: Drawing rounded corners

GuiRootWidget now has a general purpose texture for drawing rounded
corners with. VertexBuilder can be used to set up a flexible
(stretching) frame that uses a texture for its corners.