bug#36894: Restore frameset focus and selectedness

[martin rudalics]

> Juri suggested a patch.  Are you okay with it,

Okay with it.  It should not make things worse.

> or are you saying that
> it will still fail in some situations?

Focus related problems are generally unpredictable.  Each window
manager under GNU/Linux and each Windows version introduces its own
ones.  Just for example, here on Windows 10 it's so far completely
incomprehensible for me to tell whether a window A hidden by a window
B will get focus when B is deleted.

martin