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 LP: #878516 and also LP: #925867 as a side effect. Fixes: https://bugs.launchpad.net/bugs/878516, https://bugs.launchpad.net/bugs/925867.

Approved by Daniel van Vugt, MC Return.