Document/decide on dynlib tag for import libraries

If you have a module/rule or similar that somehow produces a dynamic library (e.g. building it with qmake) that your other products depend on, you need to tag the ouput library such that qbs adds it to the librarires that should be linked in.

By reading the qbs code, I found that dynamiclibrary_copy was the correct tag to use, but the name implies something internal, and also it isn't documented. kandeler suggested on the mailing list that the file tag maybe should be dynamiclibrary_import for such cases.