Insert prototype data on existing entities

[MrGVSV]

Objective

Add ability to insert prototype data on existing entities.

Solution

Added default trait method to Prototypical called insert, which takes an EntityCommands object. Its usage looks something like:

fn attach_components(entity: &mut EntityCommands, data: &ProtoData, asset_server: &AssetServer) {
  let proto = data.get_prototype("My Prototype").expect("Should exist!");
  proto.insert(entity, data, asset_server).insert(SomeOtherComponent);
}