Save entity as prefab only

[naomijub]

Currently we can only save a whole scene as a prefab, and when we load a prefab we load the whole scene deleting the old one. We need a method to save entities as prefabs that can be spawned in a scene.

Bot_menu save should save all entities and load should load a complete scene.

We need a way to right click an entity in the entity hierarchy, save it as a prefab and load it as a new entity in another scene

[naomijub]

Relates to #102

[naomijub]

Duplicated with older issue #66