Grid: Correctly restore semi-maximized windows.

Changes include:
1) Remove the special status of fully maximized windows - it just adds
   complexity to the restoration process. Core can handle the
   restoration of maximized windows perfectly well.
2) Catch un-maximation of a gridded semi-maximized window in
   stateChangeNotify and call restoreWindow to revert the changes.
3) Refactor restoreWindow to work with semi-maximized windows.

This fixes bug 878516 and also bug 925867 as a side effect.