Reverse-compatibility with v20

[LakeYS]

Some players have shown interest in maintaining compatibility with version 20 of Blockland, In theory, we could unify the add-on to work in both v21 and v20. A similar thing was done in the Renderman mod and it worked well. One thing to note is that in order to do this, support for RTB's old pref functions will need to be maintained.

The game's version can be checked using the $Version var.