Implement shim legendary classes with overloaded/modified functions

[loathingKernel]

Implements shim LegendaryCore, DLManager and UIUpdate classes with the modifications required for legendary.

This can enable Rare to use system-provided legendary instead of the submodule. Updating these classes can be taken care of by rebasing the forked legendary and copying over the related methods. This work can be expanded to override even more things we might require from legendary, but I would prefer to keep it at a minimum in general.

[loathingKernel]

I think this can be considered to be merged now if there isn't anything more we would like to add before merging. There is still more work to be done in this direction but nothing directly depends on it currently.

[Dummerle]

I will test it in the evening. Great work, thank you!