[eml] allow to bind to eml-based assets

[jkb0o]

After #5 (and may be #1 ) will be implemented, it will be possible to select elements as entities using ess selector and provide bindings for them:

// declaring eml scene
#[derive(Component)]
#[scene("path/to/scene.eml")]
pub struct ProfilePage {

 #[source]  
 pub profile: Entity

 #[select(.profile-avatar)]
 pub avatar: Entity
 
 #[select(.profile-name)]
 pub name: Entity

 #[select(.profile-health)]
 pub health: Entity
}

// handle loading
impl SceneBind for ProfilePage {
    fn bind(&mut self, this: Entity, commands: Commands, asset_server: AssetServer) {
        commands.add(from!(this, Self:profile @ User:name) >> to!(self.name, Label:value));
        commands.add(from!(this, Self:profile @ Health:current) >> to!(self.health, Range:value));
        commands.add(from!(this, Self:profile @ Health:max) >> to!(self.health, Range:maximum));
    }
}

Later we can all required information for instancing scene:

fn setup(mut commands: Commands, asset_server: Res<AssetServer>) {
  let player = commands.spawn_empty().id();
  let scene = asset_server.instance::<ProfilePage>(player);
  commands.add(scene);
}

[jkb0o]

This will be done right in polako (#77)