Allow for simple definition of title and description

[Citrullin]

If I understand it correctly, the definition of title and description is at the moment always derived from the default language. However, there might be cases where you want to reuse a thing model that has a predefined title or description and no language specific titlesor descriptions.

I would therefore suggest that we offer the possibility to define both title and description on their own with the current (very nice) solution as a potential fallback.

Also (partly) fixed by 02a1de4: title and description are matched with/inserted as the field for the defined @language ("en" if @language is not defined).

@Citrullin: You've mentioned that you've intended to allow to switch the default language at runtime. I guess to allow for that we need some kind of mechanism to ensure that this language will actually be defined at runtime.

Maybe we could define the supported languages in the Makefile which are then passed on to the python script. There we could check if all titles and descriptions contain all specified languages.