Osu: Add the ability to transfer custom rullsets when updating the application

[Moltanicaa]

When updating, they are not saved and are in the folder with the old patch

Proposal designs of the feature:
I think that in the settings you can add a transfer button. Or enable automatic transfer

[Game4all]

One way to tackle this would be to create a Rulesets folder in the game data directory where the custom Rulesets assemblies are put and so aren't located at a path dependent of the game version. Or to have Rulesets databased like every other game content.

[RoARene317]

or the other selection is by overwriting the file when restarting the game.

[peppy]

Storing to the database may be the final goal, but for now I believe a rulesets folder is best (as we may be changing database engine in the near future and are avoiding making any large database changes).