Gravity!

This update fixes a few more bugs (there were a few updates to solid actors that I forgot to add to moving platforms), but the main addition is that it now adds gravity.

There are two new events that you can use on actors to enable gravity or disable it. This is an experimental feature, so it may break in unexpected ways!

Gravity is also contained in it's own plugin folder, so if you want to remove it from your game while keeping the other P+ features, it's easy to delete it.