main: Cancel thumbnailing on destroy

Stop thumbnailing when TotemGrilo object is destroyed.
main: Make it possible to pause icon thumbnailing

Using g_thread_pool_set_max_threads() with 0 max threads, to freeze
further processing until we reset the max threads count.
main: Add helper to pause thumbnailing in grilo widget
main: Pause thumbnailing when playback starts
backend: Add track-skip-requested signal

So that the video playback can request skipping tracks.
backend: Fix object type in signal handler

It's a ClutterSwipeAction, not a ClutterTapAction.
backend: Request next track when swiping with 2 fingers

https://bugzilla.gnome.org/show_bug.cgi?id=697896
main: Skip tracks when using 2-finger swipes

https://bugzilla.gnome.org/show_bug.cgi?id=697896
main: Handle "Powerpoint" buttons on remotes

Some remotes, including the Logitech Presenter R800:
http://www.logitech.com/en-us/product/professional-presenter-r800-business
have a number of buttons bound to make it easier to use Powerpoint
presentations.

We bind some of the buttons used on those presenters to functions
in Totem to allow starting/stopping playback and seeking.

https://bugzilla.gnome.org/show_bug.cgi?id=704608