Cleanup and update of `gamemode` commands

Updated gamemode console and in-game commands to behave in a more uniform manner, centralized into gm.lua to facilitate reuse of common code, updated Info.lua and README.md with additional information.
I'm not sure if you understood the `ParameterCombinations` right. They don't list individual parameters, they list combinations of them. Each item is a separate usage of the command.

What your description says now is that you can use either of these commands:
```
/gm s
/gm xoft
```
However, it says nothing about what the second one will do - there's no gamemode to be set for xoft in the parameters.