mediascanner add mode "incremental scan" = scan only for new/deleted photos

[k3b]

Currently the media scanner does a full scan starting at the selected directory which is quite time consuming if you have lots of photos.

The incremental mode will only scan photos that

• are not in the media database yet or
• that have beeń added by the android native media scanner but were nevers scanned by APhotManager's own mediascanner (which adds infos IPTC and XMP)

This ticket is part of #169