[TASK](ui): Harmonise event handler return values

[franzheidl]

We have inconsistencies in the return values of various handlers. Some return the event, some a value, some synthetic objects fo multiple values. These are historically grown, and in some cases just forwarded from what a third party lib returns. We should assess the current situation, decide whether there is a single approach we want to follow, or if (and why and in what cases) we allow inconsistencies. We should aim at being predictable and practical for our users.

In order to address the problem, we should do the following:

1. Collect and document the current return values of all components that accept handlers
2. Assess the collection and come up with a recommendation how to harmonise these
3. Implement harmonised event handler return values