Geometry allocate signal and meta window get minimum size

In EndlessOS we had some downstream changes on mutter that allow us to sync two windows sizes. We are removing as much downstream code as we can to try to make EndlessOS desktop as much upstream as possible and this was removed during that cleanup process.

I'm creating this MR because maybe it's interesting to have this upstream instead of loose this functionality completely.

This Merge Requests bring back two commits. The first one adds the API to listen and modify the minimum size of a window. The second one adds a function to get the minimum size for a meta window.

We used this on the hack experience, to sync the size of two windows correctly. Right now, we can't do this correctly, and there are some cases where the sync is not correct, but it's not an important issue, because those cases are not common user cases. In any case, having this will improve the sync window experience.