Features/inspection mode

[BastianHavers]

Add inspection mode

In inspection mode, two types of problems are automatically detected:
object ids that only occure for a small number of consecutive frames
(often thede detections are wrong),
and double detctions (overlapping bounding boxes, which are often wrong, too).
Inspection mode is activated via the "Edit" menu.
In inspection mode, one can jump between problems with arrow keys, or do
nomral editing. With a button, inspection mode can be left again.
In future, maybe more things could be detected in inspection mode.