xwayland: Update net_wm state when we change it.

According to the wm-spec we must keep the _NET_WM_STATE property updated
to reflect the current state of the window.

This has been biting me when firefox starts maximized, then I click the
maximize button to toggle to unmaximized state. The next time I mouse over
the maximize button (which causes the frame to be re-rendered with the
maximized button in a highlighted state) we re-read the window state and
weston then believes the window is maximized even though it is being
rendered in a not-maximized state.

Update the state when we change maximized status so this doesn't happen.

Signed-off-by: Derek Foreman <derek.foreman@collabora.com>