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-frontend/src/app/app.element.ts

@@ -189,8 +189,7 @@ export class AppElement extends LitElement {
 				nothing
 			}
 			<sts-viewer
-				objectId="base-human-female"
-				skyboxId="test_scene"
+				objectId="Mannequin.gltf"
 				.mode=${this.mode}
 				@scene-state=${this.onSceneStateChange}
 			></sts-viewer>