The addition of _backgroundStack to ModalDialog broke focus
navigation, because it was interposed between the focus group root and
all of the interesting content, but since it isn't an StWidget,
st_widget_navigate_focus() was unable to navigate through it. Fix this
by moving the focus root to this._dialogLayout (inside the
_backgroundStack) instead.
Additionally, in EndSessionDialog specifically, _initialKeyFocus
wasn't being set until after opening the dialog, so it was ignored.
Also change ModalDialog.close() to clear the _savedKeyFocus that
popModal() set, so that dialogs that are repeatedly closed and
reopened will have their focus revert back to _initialKeyFocus each
time.
https://bugzilla.gnome.org/show_bug.cgi?id=646740
59c3e3a179