This applies for all project managers, except qmake. The qmake project
manager will make use of this in follow up changes.

Before, "foo.h" was always recognized as a CXXHeader. Now, it depends on
the other files. E.g. in a file list {"foo.h", "foo.c"} foo.h is now a
CHeader. In {"foo.h", "foo.c", "bar.cpp"} the file "foo.h" is ambiguous
and we will create two project parts, one where it is a CHeader, the
other where it is a CXXHeader.

Change-Id: I50505163368742584b1380c284d42cbe07cb4fc9
Reviewed-by: David Schulz <david.schulz@qt.io>