[Enhancement][Metadata Utilities] Add items when (combination of?) other values exist

[Jeshibu]

Consider you've got the tags First-person shooter and Third-person shooter, and you want those to automatically add the genre Shooter to a game.

Another use case for combined requirements: you've got genres Perspective: Third Person and Shooter and you want that to add the genre Third-person shooter

[HerrKnarz]

I thought about how to implement that. Here's a rough idea:

• support four condition types:
  • All conditions are met
  • None of the conditions are met
  • At least one of the conditions is met
  • execute only manually
• a rule can have any number of conditions. A condition can be the following:
  • field xy = abc (also support fields like library here)
  • field xy is empty
  • field xy != abc
• a rule can have any number of actions to take. An action can be the following:
  • add value ABC to field XY
  • remove value ABC from field XY
  • clear field xy (will be executed before adding stuff to that field of course)

These rules can then be set to be executed after metadata updates or manually. The latter should also have the option to ignore the condition, so you could have the same rule for automatic execution and manually for games you select yourself. This concept thus also covers #96, so that it can be shelved.