opensubtitles: Save downloaded subtitles in the video directory

Try to download the subtitle to the movie's directory, and to
the cache directory if it fails (read-only locations)

https://bugzilla.gnome.org/show_bug.cgi?id=721757
main: Fix possible warnings on startup

If a media isn't set in the store, skip the comparisons
main: Block popup hiding when "Go" popover is shown

When the "Go" button is toggled on and the menu is shown, block
hiding the popup.
main: Block popup hiding when "Volume" popover is shown

Ditto but for the Volume popup.