main: Rename "Recent" section to "Videos"

The recent section shouldn't be called "Recent". We
chose not to show recent videos and show the user's videos instead,
along with the locally download ones, and optical media.

"Videos" is simple, and already translated. It also matches what
gnome-photos uses for its equivalent view.

https://bugzilla.gnome.org/show_bug.cgi?id=725337
main: Only resolve using the passed source

As originally intended, rather than going through every online
sources that might be available.
main: Fix handling of kbd shortcuts with multiple modifiers
main: Blacklist adult sources for now

Until we have a toggle for it in the Privacy settings.
main: Remove unused RESOLVE_FLAGS
main: Fix ngettext usage

https://bugzilla.gnome.org/show_bug.cgi?id=725401