videolibrary: add choice to resume video when using "Play using..." (fixes #14358)

This change adds the possibility for the user to choose whether to resume a video or start from the beginning when manually choosing the player through "Play using...". It makes use of the "myvideos.selectaction" setting and either automatically plays the movie from the beginning, resumes it or asks the user whether to resume or to start from the beginning (same as when playing the video with the default player). Because the other possible values of "myvideos.selectaction" don't make any sense (the user has already decided that he wants to play the video) they are replaced with the SELECT_ACTION_PLAY_OR_RESUME option.

This has been mentioned/requested in http://trac.xbmc.org/ticket/14358.