Handle window maximization in separate callback

The old code queried the maximized state from within the window
resize callback configure_event, getting a stale value sometimes
( https://bugzilla.gnome.org/show_bug.cgi?id=724951 ).

The correct approach is having a separate
window_state_event callback to handle maximization.

Test case for Geary main window:
1) Maximize
2) Unmaximize
3) Close
4) Re-Open
Before patch: Opens maximized
After patch:  Opens unmaximized
(Or just watch the output of "dconf watch /org/yorba/geary/")