Editor: paste entities at the cursor

Created by: christopho

When using copy-paste in the map editor, it would be better to paste entities at the cursor location. And even better, to let them float and follow the cursor like entities that are being added.

This feature is crucial to ease copying entities from a map to another one.

Copying entities from a map to another one is very useful, especially when you use a special map to store frequently used pattern of grouped entities, like trees, cliffs, walls, stairs, doors, etc.