Improve multi-value features

[reckart]

Is your feature request related to a problem? Please describe. Improve the support for multi-value string features, in particular in combination with recommenders

Describe the solution you'd like

• [ ] Recommenders: When a recommendation is on a multi-value feature, do not hide overlapping suggestions (same as when stacking is enabled)
• [ ] Recommenders: When accepting suggestion for a multi-value feature, merge it into potentially existing annotation, unless stacking is enabled
• [ ] Recommenders: Check if active-learning works with the multi-value feature