CppTools: "Switch Header/Source" checks also other projects

...and not only the current one as fallback.

E.g. if the projects qtcreator and qtbase are open and you navigate into
qdir.h of your include path from the qtcreator project and trigger
"Switch Header/Source", then "qdir.cpp" will be found in the qtbase
source dir.

Task-number: QTCREATORBUG-3789
Change-Id: Ibd73c37bb626f8f7ee80cd8be3ef1244883ccf5e
Reviewed-by: Erik Verbruggen <erik.verbruggen@digia.com>