Implement window gravity support in WindowManager.reposition()
Rewrite move-to-center to use the new gravity support.
Silence Glib's complaints about different enums in the same dict.
Filter the spurious libwnck messages from QuickTile's output
Improve generated API docs
Typo fix revealed by unsetting "col" when it's no longer needed.
Remove DependencyError.

(With the code reworked, it's pure overkill as a solution.)
Apply staticmethod and classmethod decorators as appropriate.

(Since the internal API isn't frozen yet, there's no downside and it helps
to make it more clear which operations aren't intended to have side-effects
 ...which will help come test suite time.)