Make GWindow::close() so we can override it in GDialog and quit from the
internal event loop when the window manager tells us to close ourselves.
The dialog will return GDialog::ExecCancel in these situations.
This behavior and API was extremely counter-intuitive since our default
behavior was for applications to never exit after you close all of their
windows.
Now that we exit the event loop by default when the very last GWindow is
deleted, we don't have to worry about this.
