Add layer merging tool

[TimRiedel]

As a user I would like to have the opportunity to see the overlap of multiple annotations based on the intersection of annotations and reviewer consensus. I also want to merge annotation layers.

Description

Acceptance Criteria

• there should be a merging tool
• clicking the merge tool should open a merge popup
  From within the merge popup:
• it should be possible to select only certain layers for merging (e.g. "5 layers currently selected >" opens another popup that allows to select the layers)
• existing annotations should not be amended --> a merge works on / creates a new annotation
• the annotation consensus should be adjustable by a slider (show only the regions with x overlapping annotations)
• confirming the merge creates a new annotation and closes the popup

[TimRiedel]

See also PR #337