Support drag-and-drop for sensible multi-selections

[abey79]

Currently we support drag-and-drop of single-selection only (we force-single-select on drag start). We should support drag-and-dropping multi-selections at least when they "make sense", e.g. only entities are selected, or only views and containers are selected.

TODO:

• provide some kind of feedback of the "draggability" of the current selection
• alternatively, keep the current force-single-select behaviour when the current multi-selection is not draggable

Related:

• Drag-and-drop: add support for view hierarchy item multi-selection drag-and-drop #5307
• Enforce homogenous multi-selections #4887