GnomeIdleMonitor: use the new Mutter API instead of raw X calls

This way we can track the user activity on wayland too.

https://bugzilla.gnome.org/show_bug.cgi?id=706006
Updated Hungarian translation
build: Add idle-monitor.xml to EXTRA_DIST
build: Bump soname for GnomeRR* ABI breaks
Tajik translation updated
GnomeRR: read the backlight value after asking mutter to change it

Mutter now returns the new backlight value after a successfull call
to ChangeBacklight, so we need to read it.

https://bugzilla.gnome.org/show_bug.cgi?id=706729
GnomeRR: Do not assume outputs with the id of 'default' are built-in

We are using this in all kinds of places to work out if a display is embedded
(like a laptop display) and cannot be removed. Some of the binary drivers do
not set a sensible ID, but it's not more sane to assume these default values are
either for internal or external panels. Assuming the former breaks VMVGA in
virtual machines. We should not return TRUE when there is no heuristic to use.

Resolves: https://bugzilla.gnome.org/show_bug.cgi?id=706738
Updated Lithuanian translation
Update Kazakh translation
Added Galician translations for GPL license
3.9.91
Updated Serbian translation
IdleMonitor: protect watches from being freed while in flight

If you added an idle watch and then immediately removed it, it
was possible that the watch was freed before the dbus call was
handled by mutter, thus causing a crash later one.
Prevent that by reference counting the watch structures.

https://bugzilla.gnome.org/show_bug.cgi?id=707396
Updated Russian translation
thumbnailer: Always print errors from gs_file_read_noatime()

https://bugzilla.gnome.org/show_bug.cgi?id=699252
thumbnailer: Correctly check for errors

By checking the retval, not whether error was set.

https://bugzilla.gnome.org/show_bug.cgi?id=699252
thumbnailer: Check gdk_pixbuf_loader_close()'s retval

https://bugzilla.gnome.org/show_bug.cgi?id=699252
Updated Traditional Chinese translation(Hong Kong and Taiwan)
Updated Italian translation
Finnish translation update by Jiri Grönroos