Refactor Studio entry-points (#54)

This PR heavily refactors the entry-point functions in
`studio/src/lib.rs` to simplify and consolidate. Additionally, the
frontend application should now be configurable as expected with any
valid combination of the following URL parameters:

* `objectId` : A pre-bundled library object to load (e.g., "couch.gltf")
* `bvh` : Any resolvable path to a BVH animation with a MocapNET
skeleton (e.g., "mocap/shuffle.bvh")
* `mixamo` : Any resolvable path to a glTF animation with the Mixamo
skeleton (e.g., "mocap/hip-hop-dancing.gltf")
* `sceneImport` : Any resolvable path to an arbitrary glTF scene
(untested, but should theoretically work)
* `scene` : Any resolvable path to a previously saved Storyteller Studio
scene in `*.scn.ron` format
* `skybox` : The ID of a pre-bundled skybox asset. Valid options are:
  - "gum_trees_4k"
  - "kloofenfal_28d_misty_4k"
  - "meadow_4k"
  - "promenade_de_vidy_4k"
  - "scythian_tombs_4k"
  - "test_scene" (this is the default if not specified)
* `mode` : One of "studio" or "viewer" (the default if not specified)

"Validity" rules are as follows:

* `objectId`, `bvh`, `mixamo`, `sceneImport` and `scene` are mutually
exclusive
* `skybox` is not supported for `scene` (the skybox should be saved and
loaded as part of the scene description, though this is not currently
working)
* Both `mode` options are valid for all other parameters

studio/src/main.rs

@@ -1,6 +1,9 @@
-use bevy::{app::Update, window::close_on_esc};
+use bevy::{
+    app::Update,
+    window::{close_on_esc, Window},
+};
 use clap::Parser;
-use studio::{screenshot::ScreenShotPlugin, InitSeed, StudioMode, ViewportSize};
+use studio::{screenshot::ScreenShotPlugin, InitAsset, InitParams, StudioMode};
 
 #[derive(Parser)]
 #[command(name = "StorytellerStudio")]
@@ -20,15 +23,16 @@ fn main() {
     let skybox_id = args.skybox.unwrap_or("gum_trees_4k".to_string());
 
     // Edit the arguments here to change the default startup scene or skybox
-    let mut app = studio::start_internal(
-        InitSeed::FromLibrary {
-            object_id,
+    let mut app = studio::start(
+        InitParams::FromAsset {
+            asset: InitAsset::LibraryObject(object_id),
             skybox_id,
         },
-        "", // ignored for non-WASM
-        "Storyteller Studio",
-        ViewportSize::default(),
         StudioMode::Editor,
+        Some(Window {
+            title: "Storyteller Studio".into(),
+            ..Default::default()
+        }),
     );
 
     // Add any additional systems, resources, etc. for local debugging