No-holes functionality for custom tiles

[DeadMG]

I've been adding some custom tiles and one thing that would be nice is support for the no-holes feature that space platforms have.

In terms of how this would work APi wise, it can probably be supported as a generalisation of the current logic for space platforms. The modder specifies an entity prototype on the tile prototype, and when the player attempts to place these tiles, the tiles must be connected to tiles which have that entity on them, otherwise it's refused. This should mean that the platform foundation can get the current behaviour by specifying the space platform hub in this role. Then it's up to the modder to spawn the tiles/entity as appropriate and handle what if the entity gets destroyed, etc.