Mergeing multiple brush annotations

Problem:
ML backends (e.g. Segement Anything) can sometimes propose annotions that only cover an object partially. The model can be prompted interactively to get an annotation for the remaining part of the object. However the object is now covered with two masks, which does not work in a setting where each mask should relate to one object instance.

**Describe the solution you'd like**
A merge two brush annotations option for selected masks could solve this problem.

**Describe alternatives you've considered**
The API call alternative to this problem is very user unfriendly.