main: Add async thumbnailing helper

To thumbnail videos using totem-video-thumbnailer through gnome-desktop,
using a GThreadPool to make sure we don't request more than 5 thumbnails
at a time.

https://bugzilla.gnome.org/show_bug.cgi?id=727748
main: Port thumbnail fetcher to GTask

And make sure we don't simply return from a failed icon fetch!

https://bugzilla.gnome.org/show_bug.cgi?id=727748
main: Fetch missing thumbnails

When populating the icon view.

https://bugzilla.gnome.org/show_bug.cgi?id=727748