do the hide-is-move dance w/ desktopgrid buttons

the (old) button effect windows used to be unreferenced
with the re-invocation of the effect. because we stopped
deleting/recreating the window, this approach failed and
the effect window was never deleted.

Unreferencing the window at proper occasion (see new hide
location) coked up - guess what - the exact same "texture is
junk" issue as for the QtQuick close button in present windows...

So we resort to the exact same stupid "hide by moving" solution
as we have there.

BUG: 348577
FIXED-IN: 5.4
REVIEW: 124136