Correctly locate files with relative paths

Correctly locate files with relative paths

QUrl("relativefilename").toLocalFile() changed behavior
and now returns an empty string if the scheme is not set.
Setting the scheme to "file:" in setSource would however
break some other assumptions in the code about relative
url's.

Task-number: QTBUG-22416
Change-Id: I1b3fcbef81f6e356935ec426903989e783ce9a78
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>