Implement mount/unmount user interaction in Cinnamon.

Certain aspects of GtkMountOperation are broken under Wayland in
Gtk3 and won't be fixed. Fortunately it can also use a dbus
interface (org.gtk.MountOperationHandler) if a provider exists.

This allows us to:
- provide the 'device is in use' popup and showing which
  application is using it, when trying to eject a device.
- provide the password/question dialog when mounting devices that
  require interaction before mounting.
- eliminate a lot of code and translations from placesManager,
  and make the behavior identical when interacting with a device
  whether from a file manager or Cinnamon's drives applet.
  Translations are provided by Gtk, Gvfs instead (as when
  ejecting a device from a file manager).

Author: mtwebster

docs/reference/cinnamon/meson.build

@@ -1,6 +1,7 @@
 ignore = [
     'cinnamon-recorder-src.h',
     'cinnamon-recorder.h',
+    'cinnamon-mount-operation.h',
     st_headers,
     st_private_headers,
     tray_headers,