If the default install settings contain a value for
"Settings/InstallSettings", then that is used as the actual path for the
actual install settings instead.

If the path is relative, it is interpreted as relative to Qt Creator's
applicationDirPath.

Change-Id: I212037112e88c60c94239580841aa1400d96fb73
Reviewed-by: hjk <hjk@qt.io>
Reviewed-by: Kai Koehne <kai.koehne@qt.io>