Add support for mypaint-brushes-2.0

The new upstream has already released stable 2.0. Creating this issue as a reminder, since it blocks our downstream PR.

Edit by Jehan on 2021-04-03: as explained by libmypaint devs, now we don't have to wait for libmypaint v2. The new API which makes use of v2 brushes was backported in libmypaint >= 1.5. If we implement these new API in GIMP, we will be able to use v2 brushes with no unexpected behavior. We welcome any patch doing this. Then we can move on to v2 brushes.

Edit by Jehan on 2021-04-05: the link with technical details for any developer interested to update our MyPaint implementation: https://github.com/mypaint/libmypaint/issues/101#issuecomment-813000137