Allow Alt-Category exclusively

[nji9]

Reason:
viewtopic.php?p=211388#p211388

Possible solution
Alt-Category: filter on that only (other cleared).
Alt-Ctrl-Category: as up to now (i.e. with respect to selection before)

Advantages:
Filter to all images not assigned to a category is straight&easy.
Using the additional Ctrl-key is more inline with it's so far meaning.

Drawbacks:
The actual meaning of Alt-Category will be changed.
The maybe most-used case (filter on a pre-selection) need one more key.