Clicking the button generates a WindowCloseRequest event which the client app
then has to deal with. The default behavior for GWindow is to close() itself.
I also added a flag, GWindow::should_exit_event_loop_on_close() which does
what it sounds like it does.
This patch exposed some bugs in GWindow and GWidget teardown.
Work now happens in terms of two messages:
- WM_ClientWantsToPaint
- WM_ClientFinishedPaint
This feels fairly obvious compared to the old Paint/Invalidate.
