GmWindow: Allow applications to save and restore the state of GmWindow.
GmWindow: Fixed position not being restored when the window is shown.

We fixed the code to restore the window state when it is realized. That
prevented a flickering effect to appear when starting Ekiga. However,
when the window is hidden, then shown again, the state was not restored
back. This is now fixed by also connecting to the show signal.
CallWindow: Restore state when going unfullscreen and added F11 accel.