EddyEditor: possibility of selecting and copy-pasting elements from different groups

[Krzyhau]

It might be very cool to allow a flow, where multi-selection of stuff from different groups was possible - let's say you want to move or copy-paste a structure you made, that is made out of triles, art objects and background planes. Kinda related to #81

[Krzyhau]

After further investigation, it seems that you can already do that with art objects and background planes just fine. This synergy doesn't include triles, unfortunately.