As briefly discussed in #1459, we should implement a Lua API version which is set via meta.xml.

We always want to ensure backwards-compatibility when making changes to code, however this may prevent us from:

1. Keeping code clean
2. Making "beneficial" changes (that would break backwards-compatibility)

An example of this is with CEGUI, where functionality in newer versions may differ from the version we're currently using. By default, we'll ensure everything looks/feels and functions the same as before, however this may prevent new features/changes in CEGUI being used (or at least implemented cleanly).

The server/resource should decide if they care about backwards-compatibility over improved functionality.

Another example may be when "extended" function alternatives have been suggested (#638) - instead of adding a separate function we'd simply allow them to set the API version specifically for this function, e.g:

<api name="setElementData" version="123" />

I haven't really thought about how exactly this will be implemented or configured via meta.xml, so input here would be great.