make qttranslations work for qt5

modularization of the libraries should be followed up by modularization of the translation catalogs. however, this is in direct opposition to the "minimal porting effort" claim, given that our way to include catalogs is rather explicit and bothersome.

therefore, i suggest that we introduce the concept of "meta catalogs" which pull in the real catalogs via dependencies, so qt.qm can be loaded as before without the user noticing that there are in fact multiple catalogs in the background.

we need:

• the ts format needs a list field <depends> in the "header" (a linguist gui for that is P5)
• this needs to be pulled into the binary qm format
• presumably, QTranslator needs to be able to own other QTranslators, so when it is passed to QCoreApplication::installTranslator(), the dependencies are registered as well. i'm not yet sure that this should be exposed in the api, but given the design of QTranslator which aims at being fully re-implementable, i would say yes.
• QTranslator's qm loader needs to resolve and load the dependencies recursively

when we have that, we can proceed with splitting the catalogs in the qttranslations module.