grabHelper: Introduce a stack of grab helpers

GrabHelpers use a 'captured-event' to steal events and emulate
modality or grab-like semantics. There can be issues when you try to
use multiple GrabHelpers stacked on each other. As Clutter follows
the DOM-like semantics of "first come, first serve", when a second
GrabHelper connects to 'captured-event', its callback will only be
processed *after* the first GrabHelper's callback is called.

This breaks the expectation of narrowing modality where new modals
take priority over the old ones.

Solving this globally in a cleaner manner would require a rewrite of
pushModal/GrabHelper. As a stopgap fix for now, use one shared
'captured-event' handler between all GrabHelper instances, and
delegate to the individual GrabHelpers.

https://bugzilla.gnome.org/show_bug.cgi?id=699272
messageTray: Move the tray menu to a button

https://bugzilla.gnome.org/show_bug.cgi?id=699272