libdeng2|Widgets: Improved RuleRectangle API; fixed ScalarRule destructor

ScalarRule did not leave the Clock's audience when destroyed, causing
a crash if an animation was ongoing.
UI|Client|GuiWidget: Added a blurred background with border glow
UI|Client|MenuWidget|Fixed: Layout logic for expanding unlimited columns
UI|Client|PopupWidget: Directionless anchor

NoDirection centers the popup on the anchor point.
Client|Homepage: Opening the homepage in a web browser
Cleanup
UI|Client: Added DialogWidget

Dialogs are popups with a scrollable content area and a row of
buttons (in a menu widget).
UI|Client|Task Bar: Added an About dialog; "About Doomsday" in DE menu

The "About Doomsday" menu item now opens a popup dialog showing
information about Doomsday.

Todo for later: I'm sure there's lots of more information we can
show here...