Expose GTK Window.

Closes #557. This is a cherry-pick + squash + update of what was given
in #557. The omitted commit was adding Window to the UI structure which
means we'd always have to build with Pango for Yi.UI.Common to compile
which is undesirable. An alternative solution would be to parametrise
UI by the window type.

Conflicts:
	yi/src/library/Yi/UI/Pango.hs
A little housekeeping