Cleanup|libdeng2: Arguments of Widget::notifyTree()
libgui|GLState: Apply scissors cumulatively

Each scissor cuts away from the previously defined scissor.
libdeng2|Range: Added clamp() method
libdeng2|Rectangle: Added a new contains() and intersection methods
Fixed|libdeng2|OperatorRule: Broken Double operator, added clamped

The Half and Double operators are now used when dividing/multiplying
by 2.
libdeng2|ScalarRule: Animation styles
libdeng2|Widget: Child hit clipping, notifications about parent changes
Debug|libdeng2|RuleRectangle: Debug name for RuleRectangle
Default Style: Added inverted text and background colors
Client|GuiWidget: Added virtual methods about child addition/removal
Client|UI: Added Direction to the ui namespace
Fixed|CommandAction: Missing call of base class trigger
Client|ButtonWidget: Observing the state of a button
Client|ScrollAreaWidget: Scroll area clips children
Client|MenuWidget: Menu layout that expands in both directions

Fixed or unlimited number of columns/rows.
Client|UI: Added popup widget and popup menu widget
Client|Task Bar: Opening a popup menu from the DE button

Plus some minor fixes/enhancements.
Merge branch 'master' of ssh://github.com/skyjake/Doomsday-Engine
Fixed|Client: Build failure (static class member initialization)
Client|PopupMenuWidget: Use accent color for highlighting pressed item