windeployqt: Fixed --debug and --release together with --qmldir

If --debug or --release is used, findDependentQtLibraries should not
change the isDebug value, so another variable was introduced.
"detectedDebug" gives information about the detected debug state of
the executable/library but this value might be ignored if --debug
or --release was used.

Change-Id: I2c6d4188179f30e17d054a6821e96071a6296207
Reviewed-by: Friedemann Kleint <Friedemann.Kleint@digia.com>