Handle object reference form inputs.

[milko]

Object references are tricky to handle in forms, since they contain the native identifier of an object as value, but need to present a user friendly interface. In the current system we have one object reference type, kTYPE_REF_UNIT, which is relevant to users and needs to be handled in a user friendly way.

Tags of this type should feature the kTAG_DOMAIN property which indicates what kind of unit the field references, tags that do not have this property may reference any kind of unit.

The idea is to provide an input field that will be used with a full-text search operation that will return a list of possible choices, the user then must select one of these choices.

The choice selection could be presented in the same way as the default table view, with the addition of the relevance indicator (will be subject of another issue), giving the user an idea of how relevant his/her choices are.

The other possibility is to have tags feature another property which contains a list of tags that can be used to search the required referenceusing a set of its properties.

The idea is to first implement the first case, then explore the possibility to implement the second case.