Application: use built-in GLib functions in Activate

[danirabbit]

No description provided.

[Marukesu]

Note that this has a little change in multiple windows behaviour, get_last_window() always return the last created window, while active_window is the last interacted window.

So, when there's two or more windows, the old code use the more recent open, but the new code will use the more recent focused.

[jeremypw]

I think the changed behaviour is not unreasonable and anyway only affects a corner case.