same stack

Initially mark all toolbars of MParts invisible, they will become
visible when a parts get selected.
Also mark the toolbar of a part that is added to a (rendered) stack
invisible.

Change-Id: Id66e570dc1086f83ff8b7b12654693704fdf76d0
Signed-off-by: Rolf Theunissen <rolf.theunissen@gmail.com>