libgui: Added Window base class, synced version with project version
Refactor: Preparing to move Window to libgui

It will serve as the base class for CanvasWindow.
Mac OS X: Dropped support for OS X 10.4

Also means Carbon Qt (4.6) is gone as well as PowerPC builds.
Builder|Mac OS X: Prevent building on OS X 10.5 (targeting 10.4)

The autobuilder will now skip 1.11+ builds on OS X 10.5, which has
been used for building 10.4+ packages.
Refactor|Client|libgui: Started revising window architecture

Began a large refactoring where the window architecture is
"re-imagined" with proper separation of concerns, and generic portions
are relocated to libgui. Hopefully after this window management is
finally up to Doomsday 2 standards.

In this commit DisplayMode, DDKEYs, and the persistent window state
management have been moved to libgui. Canvas's mouse and keyboard
event production has been abstracted behind Mouse/KeyEventSource,
so that instead of events they produce notifications.

Note: Not finished at all -- barely compiles on OS X, likely fails on
other platforms.