Dependencies

This pull request replaces the old dependencies repo [kungfubanana/MonoGame-Dependencies](https://github.com/kungfubanana/MonoGame-Dependencies) with a new [Mono-Game/MonoGame.Dependencies](https://github.com/Mono-Game/MonoGame.Dependencies) repo.  This does a few things:

- Moves the dependencies to a repo the team controls.
- Makes the git download smaller as we threw away unneeded old history.
- Renames the folder from Libs to Dependencies which makes more sense.

Downstream forks will only need to do a normal submodule init/update to get the new folder.  To remove the old files you will need to manually delete the `ThirdParty\Libs` folder.  Unfortunately git doesn't make this any easier.