Add Generic Mannequin (#27)

Just adding the mannequin from Scott into the main branch so that it can
be used.

- added the mannequin.gltf
- also now requiring the file extention when selecting file to load.
This is to add flexability with file selection. Let me know if this is a
bad idea.

studio/src/scene.rs

@@ -193,7 +193,7 @@ fn init_scene_loading(
 ) {
     if let Some(id) = er.read().last() {
         let name = &**id;
-        let path = format!("gltf/{name}.glb");
+        let path = format!("gltf/{name}");
 
         // Avoid trying to re-load this scene if it's already active
         if let Some(active_scene_path) =