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/main.rs

@@ -4,7 +4,7 @@ use studio::{StudioMode, ViewportSize};
 fn main() {
     // Edit the arguments here to change the default startup scene or skybox
     let mut app = studio::start_internal(
-        "base-human-female",
+        "Mannequin.gltf",
         "gum_trees_4k",
         "", // ignored for non-WASM
         "Storyteller Studio",