Update lists before adding/subtracting/intersecting them

When you do list operations, i.e. adding/subtracting/intersecting other lists with a "parent" list, SmartPlaylist currently takes the last saved titles from them.

With the next version of SmartPlaylist there will be a new behaviour:
Lists will be automatically updated (loaded and saved) before adding/subtracting/intersecting them to/from/with the parent list. Like that you no longer need to go and reload them one by one before using them in a parent list.

Some hints about list operations:

• If you do not specify any search rules SmartPlaylist will load ALL songs you have in your media library. Adding other lists to that will have no effect, of course.
• If you want to have a list, say "A", just to add/subtract/intersect other lists, you have to specify an "impossible" rule like "Genre xyz" in "A" - this gives you an empty list to which you can add other lists
• SmartPlaylist always first adds the "+" (add) lists, then subtracts the "-" (subtract) lists and finally intersects this result with the "=" (intersect) lists