Very simple movie set management: edit/clear set and add/remove set members

This PR has been created to deal with some frustration with movie sets. Essentially themoviedb dictates what sets are scraped, which is often inconsistent over time as users change the data on tmdb all the time.

This simple solution allows one to 
- clear the associated set, or allocate it to a(nother) set.
- add a movie to a newly created set
- manage a movie set from the movie set view. Calling "manage movie set" from the set's context menu opens up a dialog that allows you to add/remove movies. At least one movie has to be selected.

For the average user, this should be a vast improvement over what's currently there, and allow to rearrange some movies in sets in a more useful way.

Post review update: The warnings have been taken out and will be kept for future PR: 
a warning through yes/no dialog is provided in the following situations: 
- when clearing the set of a 1-movie set, the set does no longer list, and the only solution is to re-scrape that one movie
- when adding a movie to a set, while it was already allocated to another set, and that was the only movie in that set

@montellese, @jmarshallnz, this is still in the RFC stage, so any feedback welcome.