main: Show popup when seeking with the keyboard

So that we can know where in the film we are.

https://bugzilla.gnome.org/show_bug.cgi?id=728735
icon-helpers: Use the theme's foreground color to render the symbolics

https://bugzilla.gnome.org/show_bug.cgi?id=740776
main: Always request title-from-filename

Just in case the source doesn't want to give us that.
main: Only show videos longer than 5 seconds

https://bugzilla.gnome.org/show_bug.cgi?id=737469
main: Hide source which provide torrent URLs

As we don't have GStreamer support for them yet.

See https://bugzilla.gnome.org/show_bug.cgi?id=741063