Calendar Base

Please enhance the category-based filtering to make shure that selected event categories work somewhat exclusively even for events with multiple categories.

Use case description:
There are several categories; two of them shall be shown only to registered FE-users.

Problem: This cannot be ensured today by standard cal functionality in case that an event has more than one category assigned.

Possible solution:
Enhance logic of current "Show only events of categeory ..." and "Don't show events from category..." functionality to behave as following:
--> Event A has category c1 and c2 assigned ==> Do not show if only category c1 is allowed // category c2 is disallowed
In order to allow a smooth transition, maybe a checkbox would be suitable in the plugin's configuration to (de-)activate the new logic.
Or, a second category tree is added to select "Show events from these categories, but not if these other categories are assigned"

I think, the decision about this should be mainly driven by performance considerations.

BTW: The safest way to hide events with some categories would be to use the "Show only events of categeory ..." function since new categories need to be reviewed and added manually (somewhat a approval workflow ;-)).