Fix for compliance with Gnome 3: when hidden, loose focus (and regain it...

... when shown); fix inspired by https://github.com/Guake/guake/issues/89

In gnome-shell, if you hide guake, no other window will get selected. You will see that no window looks as active. You have to use the mouse or Alt-Tab.

This commit solves the issue. Outside gnome-shell, the two lines should simply do no harm (I tested a bit in gnome-fallback).