libshell: Added MenuWidget
libdeng2|Refactor: RectangleRule is no longer derived from Rule

RectangleRule is not really a Rule instance, it only manages a complex
set of connections between input and output rules.
libdeng2|Shell: Improvements in the widget framework
libdeng2|Shell|Refactor: Renamed RectangleRule to RuleRectangle

RuleRectangle is a better name because the class is no longer derived
from Rule, and conceptually defines a rectangle made up from rules.
Shell: Added menu items, fixed MenuWidget