libdeng2|RuleBank: Added a 'zero' rule

When requesting a rule with an empty string, RuleBank now returns the
shared zero rule.
libdeng2|Matrix: Added 2D variant of scaleThenTranslate()
libdeng2|Widget: Partial tree traversal

Made it possible for Widget::notifyTree() to abort the traversal
when encountering a specific widget.

Todo: Collect all the arguments into a single NotifyArgs struct so
it can be passed more conveniently to child widgets.
Documentation|libgui|GLTexture: Improved apidoc
Fixed|libgui|GLShaderBank: Applying values from shader definitions
Client|Default Style: Added the window background image
Client|Widgets: Blurred GuiWidget background, fine-tuning

A GuiWidget background can now be configured to be blurred. The widgets
behind the blurred widget are drawn onto a texture and blurred using
a shader.

Note about performance: each GuiWidget does the effect independently,
so it is not a good idea to use it everywhere. Particularly when
LegacyWidget is not intended to be drawn more than once per frame.

The ring zero UI background is now drawn using a LabelWidget instead
of the old UI code.

Todo: LegacyWidget is currently disabled as it messes the OpenGL state
in some way, causing other widgets to go missing.
Fixed|Client|ConsoleWidget: Sync log expansion better with scrolling
Client|Widgets: Apply color to the GuiWidget blurred background
GuiWidget: Downsample the blur more (x4)
Fixed|libgui: Clearing a Drawable

The default program was not cleared with the rest of the Drawable.
libdeng2|Animation: EaseIn and EaseOut functions were mixed up
Client|Widgets: Added BlurWidget

Simple utility widget for setting up a blurred layer in the UI.

Refactored GuiWidget's blurred background to allow other widgets
to draw a portion of it.
Fixed|libdeng2|ScalarRule: Shifting a scalar animation

The rule must be invalidated if it is shifted so that the new value
becomes effective.
Client|Console: Adjusted scrolling in LogWidget and ConsoleWidget
Client|LogWidget: Restored default background
Fixed|LogWidget: Possible out-of-bounds cache access
Client|Widgets: Restored LegacyWidget, disabled blurring

The legacy OpenGL rendering code is not compatible with the blurring
effect -- the liberal modifications to OpenGL state in the old code
mess up GLState's behavior.

Todo for later: Find out exactly where/how the old rendering code
confuses libgui-based drawing.
Merge branch 'ui-framework'

Conflicts:
	doomsday/client/src/ui/widgets/legacywidget.cpp
Merge branch 'master' of ssh://github.com/skyjake/Doomsday-Engine