Add multi-selection to the map

[lukasrad02]

To manage large scenarios easily, it would be helpful if multiple elements on the map could be selected and then moved/changed at the same time.

[christianzoellner]

should be default for trainers but only optional for trainees (for whole trainers should be able to enable or disable it)

[benn02]

See https://openlayers.org/en/latest/examples/box-selection.html it could maybe help implementing