qmake should interpret project file encoding as utf-8

after mandating utf-8 for all sources based on qt, it would be reasonable to do the same for qmake project files.

the question is how to do that best.
currently qmake uses from/toLocal8Bit everywhere, which in the absence of codec support simply falls back to latin1.
one approach is replacing all these calls with the utf8 counterparts. the problem with this is that qfile & co. will still use latin1, which will make a mess of file system access.
the alternative would be changing the local8bit fallbacks to utf8 instead of latin1. the downside is that doing this may be somewhat intrusive (especially at this point of the 5.0 release cycle) and breaks the 8-bit passthrough the latin1 approach provides (though realistically speaking, this serves only unix, as windows uses utf-16 for filenames anyway. and as we can (must?) assume that all unix is nowadays utf-8 anyway, there is simply no target group for this safe fallback). a less invasive fallback would be #ifdef-ing the local8bit fallbacks on QT_BOOTSTRAPPED.