MessageTray: clamp unexpanded notification height

MessageTray: clamp unexpanded notification height

The height of an unexpanded notification could include expanded
content if the notification has extra widgets (like actions and images),
so tweening to that cause it to expand visually.
Instead, use the height of the message tray before the restyle
as an upper bound.

https://bugzilla.gnome.org/show_bug.cgi?id=682738