Issue #2223 - Reduce IConvertableToPath and String used for properties to a single common type

[homedirectory]

Resolve #2223, #1891

Merge after #2209

[01es]

@homedirectory, does this really resolve https://github.com/fieldenms/tg/issues/1891?

[homedirectory]

@01es, Yes, all requirements of #1891 are implemented:

1. critCondition() supports both Strings and meta-models.
2. CriteriaReflector.critName() supports both Strings and meta-models. (not sure if I fully understand this item, so correct me if this is not enough)