Refactor|Widgets: Conditional tree notification, pre/post handlers

Improved the widget tree notification mechanism to include an optional
condition callback and pre/post handlers to be called before/after
notifying the children of a widget.

Classes derived from GuiWidget now need to override drawContent()
rather than draw() because the latter contains additional logic
common to all GuiWidgets.
Refactor|libdeng2|RuleRectangle: No need to have a public Clock observer
Fixed|Client: Use of tabs in the 'version' command
Client|Widgets: Added basic implementation of ScrollAreaWidget
Fixed|Windows|Installer: Deploy the 'gui' Doomsday Script module

The previously missing 'gui' module is now deployed in the Windows
installation.