Now it will only consider C and C++ files as source files by filtering
on mime-type. This seems appropriate for now since it currently only
supports C++ projects anyway.

Also removed the list of generated files, which was always empty since
there is no way to tell the generic project which files are generated.

Task-number: QTCREATORBUG-5451

Change-Id: I12209995db4b30643af1d51123813c24d016f44c
Reviewed-by: Daniel Teske <daniel.teske@digia.com>