WindowServer: Always update the maximize button icon when we should.

We were only updating it in the WSButton callback, not when changing the
maximized state by calling WSWindow::set_maximized().

Fixes #119.

Servers/WindowServer/WSWindow.cpp

@@ -151,6 +151,7 @@ void WSWindow::set_maximized(bool maximized)
     } else {
         set_rect(m_unmaximized_rect);
     }
+    m_frame.did_set_maximized({}, maximized);
     WSEventLoop::the().post_event(*this, make<WSResizeEvent>(old_rect, m_rect));
 }