telepathyClient: decline dispatch op when approving unsupported channel type

It shouldn't happen in theory but best to be safe than sorry.

https://bugzilla.gnome.org/show_bug.cgi?id=677457
telepathyClient: ignore invalidated channels

There is a race if a channel is invalidated during its preparation: the
'invalidated' signal is already emitted so the Shell will never notice.
We fix this by simply checking if the channel is already invalidated when
receiving it from telepathy-glib.

In the approving case, we reject the full ChannelDispatchOperation as we only
support approving one channel at the time.

https://bugzilla.gnome.org/show_bug.cgi?id=677457
popupMenu: bypass changeSignal callback if action is already handled

https://bugzilla.gnome.org/show_bug.cgi?id=676447
popupMenu: do not overflow the currentItems

Do not overflow currentItems array. If the Menu section is filled
via model items-changed callback the position in the model passed to
the RemoteMenu _modelChanged can be a to be added asynchronously
action-added. Thus the item does not yet exists in the currentItems.

https://bugzilla.gnome.org/show_bug.cgi?id=676447
mount-operation: fix exceptions when showing password entry

This is a fallout from some changes in MessageTray.Source, which now
requires either defining the iconName/iconType properties on it, or
implementing createNotificationIcon, and we're not doing any of those.
Fix it by storing the gicon of the source object and using a helper
method to create the icon actor on demand, to avoid any case when the
same actor might be added twice to different containers.

https://bugzilla.gnome.org/show_bug.cgi?id=678428
Assamese translation reviewed
Assamese translation reviewed
Assamese translation reviewed
Assamese translation reviewed
Bump version to 3.4.2

Updated NEWS