Update version numbers

After going through so many experimental builds, I believe the versioning scheme I adopted is not so great.

Using semver terminology, it should be:

Major: matches TFP alpha major version (21 in Alpha 21.3) on which the mod was developed
Minor: matches TFP alpha minor version (3 in Alpha 21.3) on which the mod was developed
"Patch": major version of the mod (starting with 1 for first public release) - any changes to the mod that require a new game, must require a new major version
Extension: minor (backwards-compatible, non-game-breaking) version of the mod

Examples:

The mod is developed on alpha 21 build 324, which is the first stable version of alpha 21. This is the initial release of the mod. Version:
```
21.0.1.0
```
The mod is developed on alpha 21 build 666, which is the first build of alpha 21.1. The mod was initially released and is at 20.0.2.1, but it requires changes because of the game update. However, if the new alpha doesn't break game saves, installing the new version of the mod also will not break game saves. New version:
```
21.1.2.1
```
The mod is developed on alpha 21 build 999, which is the second build of alpha 21.2. The mod was initially released, but there is an added feature that will work without starting a new game. Version:
```
21.2.1.1
```