libdeng2|Widget: Added behavior flag for marking widget 'unhittable'
Refactor|Client|Widgets: Added common GuiWidget background geometry

All widgets can now use GuiWidget::Background to specify what kind of
a background should be drawn for the widget.

Also added a virtual hit test method to GuiWidget.
Fixed|Client|ConsoleWidget: Enable page keys when log has been expanded
Client|ButtonWidget: Check for mouse hover, react with an effect