Make Button class purely about adding visuals, and use ShellButtonBox
for behavior. API equivalences:
shell.button => shell.actor [for consistency]
staysPressed parameter to constructor => replaced by manually setting
the 'active' property of button.actor as appropriate
pressIn/release => button.actor.active = true/false
enter-event/leave-event signals => button.actor notify::hover
Along the way, this fixes a bug with the user status menu where it was
not getting set to active because the button was getting a leave
(triggered by the menu popping up and grabbing the pointer) before for
button release, which disabled the staysPressed behavior.
Reported by Michael Meeks
http://bugzilla.gnome.org/show_bug.cgi?id=593471
5803aa7e65