Fix scanning support:

* Pass scanned QImage through proper converter to QPixmap instead of through QPixmap::QPixmap(const QImage&)
- Split kpDocument::convertToPixmapAsLosslessAsPossible() out of kpDocument::getPixmapFromFile(), which wraps kpPixmapFX::convertToPixmapAsLosslessAsPossible() and returns meta information.
- Also make kpDocument::getPixmapFromFile() call removeTempFile() as quickly as possible, instead of being called from lots of return paths.

* Write meta info of scanned QImage to kpDocument