libdeng2|Record: Member copying behavior

It is possible to either copy all members or just the non-underscore
members.
Refactor|libdeng2|Info: Flagging attribute elements as such

Also warning about duplicate keys in a block.
libdeng2|ScriptedInfo: Inheritance and groups

Changed the record inheritance element to "inherits".

Group inheritance can be used for indicating that all members of the
group inherit a certain record.

Improved the apidoc.