[META] Add a setting to only allow unique selection

Problem/Motivation

Currently, Entity Browser (e.g. view) allows selecting the same entity more than once. For many cases, this is unexpected and unwanted.

For cases where this is expected for any reasons, the option should be kept, but for typical cases and future installations, items should not be selectable more than once. Users clearly struggle with that.

This is a META issue because there are and have been several issues for that already, that should be taken into consideration. See child issues.

Steps to reproduce

Proposed resolution

Remaining tasks

User interface changes

API changes

Data model changes