Add function to disable a camera via API

[petergerten]

Currently all configured cameras become active as soon as they are used.
They will stay active even if they are not used and degrade the performance unnecessary.

In interactive mode they can be toggled on and off via key - this is not the case in headless mode or if the API is used.

There should be a function to deactivate a camera (and maybe to activate - or it activates automatically when an image is requested).

Related: #2389