Fix Map Focus when Resizing Window

Resolving https://bugs.openmw.org/issues/1803

Map now appears to retain focus on wherever it was last moved when resizing the map window.
You can instead listen to the window's resize events, which removes the ambiguity:
<pre>
        static_cast<MyGUI::Window*>(mMainWidget)->eventWindowChangeCoord += MyGUI::newDelegate(this, &MapWindow::onWindowResize);
</pre>