Jobs: add non-overloaded signal "mimeTypeFound" to deprecate "mimetype"
For less need of QOverloads in the future on connects.
Name of new signal taken from existing KIO::OpenUrlJob::mimeTypeFound. Usually it makes most sense to look at the API dox (draft) for getting inspired for proper terms, so another option would be mimeTypeDetermined instead. More chars to type and read as developer, so I lean towards mimeTypeFound as recently introduced with KIO::OpenUrlJob.