grabHelper: Rework grabbing code to properly handle stacking

grabHelper: Rework grabbing code to properly handle stacking

When Dan Winship wrote the GrabHelper code originally, it didn't
handle a grab stack. I wrote the grab stack code hastily when landed
the message tray, not understanding all of the code that was involved
here.

Fix it so that we properly do the operations for each type of grab
when we first need to, and not sometimes when the first grab is taken.

https://bugzilla.gnome.org/show_bug.cgi?id=683546