gtkplacessidebar: remove notification handling

gtkplacessidebar: remove notification handling

We were notifying when an unmount operation was performed. However,
creating notifications from the gtk+ library is not that expected, and
makes notification handling difficult to do from the application point
of view since we cannot dismiss those notifications.

This cause issues like notifications of unmount drives stay there after
a system reboot, which confuses the user.

Instead of that, remove the notification handling for mount operations
on gtk+ and instead create a new signal on the gtkplacessidebar in order
to inform applications using it about an operation about to start.

Only drawback about this is that the GtkFileChooser loses its
notifications when unmounting, that although we could use the new signal
to do it, we actually don't want to notify from any part of gtk+ for
now.

https://bugzilla.gnome.org/show_bug.cgi?id=753351