main: Extend find_media_cb() to be more flexible

Instead of always comparing IDs, allow the type to compare to be
passed to the helper.
main: Don't allow adding duplicate items to recent

https://bugzilla.gnome.org/show_bug.cgi?id=729775
backend: Force reading from path for CDDA URIs

Otherwise we'll try to use the GStreamer CDDA plugins, instead of
reading from the GIO plugin.

https://bugzilla.gnome.org/show_bug.cgi?id=720120
main: Remove "all files" filters from the file choosers

So that images can't be added. Also remove audio files as a filter,
as we're a movie player, not an audio one.

https://bugzilla.gnome.org/show_bug.cgi?id=729785
backend: Reset loaded text subtitles when closing video

https://bugzilla.gnome.org/show_bug.cgi?id=728339
save-file: Videos just under ~/Videos shouldn't be copied

Not just grand-children of ~/Videos.
3.12.2