review and enhance behaviour of the search bar

[lsteinmann]

in an internal ticket (WIT-1094) and a few e-mails, people expressed their wish to search for multiple identifiers via the search bar in the style of

"identifier1", "identifier2"

in the mails in question, it was about images, but this could be generalized a bit more to allow searching for multiple values in the same style in anything that is indexed - right?

this would be a valuable enhancement of the search, but we could of course still discuss how feasible it is