Reconsider the use of the extra Qt5 namespace in the Qt5 compat module

Wrapping classes into an extra "namespace Qt5

" on top of a plain move to the Qt5Compat module requires projects using such classes not only to adapt their build systems to link against the library, but also to touch the code itself.

Please note that adding a plain 'using namespace Qt5;' in some header is /not/ sufficient to work around in case of QTextCodec.