[ATLAS] Introducing a snippet system

[Niek]

With the recent introduction of the copy-paste feature it is possible to copy entities/actors in Atlas and paste them in an text editor (XML file) and the other way around. It might be useful to provide a GUI to handle saving and loading of those snippets.

Snippets would only support things that are copy-able so entities and actors. (no textures) They could range from a completely pre-made village to small rock formations.

This feature is sometimes called POG (Premade Object Groups).

It was also suggested to add a special mode which loads a tiny "map" where you an create a new snippet.

#1213 is a requirement for this to work well.

[Stan]

Would be nice to be able to save those copy paste

[historic_bruno]

This seems strongly related to or duplicate of #2645.