add macro/lua constant for the addons versions.

[gfgtdf]

as said in the tile, it'd be nice if there would exist a way to get the current version of the addon, from lua/wml. Currently i always need to update the version in at least 2 places (server,pbl anbd in my addosn code) if i want to make a new release, i'd be nice if this could be reduced to one.

[jostephd]

Alternatively the version could be specified in WML and when uploading the value would be retrieved from there rather than from server.pbl.

[ProditorMagnus]

Splitting _server.pbl into password file and metadata file and uploading that metadata file could help.

[cooljeanius]

kinda related: #6150 (also about being able to get the add-on version)