This PR integrates asset retrieval from the Storyteller.ai back-end API,
and begins implementing long-term scene saving and storage.
### Media retrieval
Instead of fetching raw asset URLs, going forward, media files from
remote sources (e.g., in URL parameters) should be specified with a path
like the following:
```
remote://<token>.<filetype>
```
where
* `token` is a media token (e.g., "m_70b77neq9jsgmy5v31mekzchcbd223")
* `filetype` is a file extension appropriate for the type of file being
fetched (e.g., "gltf")
The `remote` protocol will be recognized by a [custom asset
reader](https://github.com/storytold/storyteller-bevy/blob/70726db111f6ccafd47feb055c5546540fef1786/studio/src/remote/reader.rs)
which will retrieve the file by calling a Storyteller API endpoint.
### Scene storage
Additionally, this PR begins laying the groundwork for
storing/retrieving Bevy scenes to/from the backend instead of solely
in-memory. This work is still incomplete -- as of this PR, a button has
been added to the web UI allowing the user to download the scene
file locally for testing/debugging purposes.
### Test URL
Media retrieval from the backend can be tested by [visiting this
URL](https://deploy-preview-49--storyteller-studio.netlify.app/?mode=viewer&mixamo=remote://m_70b77neq9jsgmy5v31mekzchcbd223.gltf)
in Chrome with the `--disable-web-security` flag (to bypass CORS errors)
This PR adds a WIP timeline UI for animating elements in the scene.
### Implemented Features
* The timeline can be panned with MMB and zoomed with Ctrl + Mouse Wheel
* The timeline can be "scrubbed" by dragging the red "play-head" with
LMB
* The timeline sidebar (displaying the list of tracks) can be resized by
dragging on the edge of the sidebar
* Keyframes can be added by pressing the "Add Keyframe" CTA in the UI
* Keyframes can be selected for editing by clicking the diamond-shaped
markers in the timeline
* Selected entities (including but not limited to keyframes) can be
deleted by pressing the "Delete" key
* Keyframes can be moved temporally by dragging the diamond-shaped
markers in the timeline
### Known issues
* Zooming the timeline in too far results in a soft-lock
* When zooming the timeline, the "transform origin" is currently
centered on the `00:00` timestamp. For ease of use, it should center on
the mouse cursor position instead.
* The timeline track-list is currently hard-coded to list the Camera and
Camera Target. Clicking on these entries currently serves no useful
purpose and may result in a crash.
* Keyframe markers in the timeline are not currently associated with
entries in the track-list and simply appear vertically centered in the
timeline view.
* Added keyframes currently record the position and orientation of the
scene camera when added, and are rendered in the world with debug
visualizations, but serve no useful purpose. Some design work is needed
(both user-experience and architectural) to decide on a workflow for
actually animating scene elements.
* Entity deletion cannot currently be undone/redone
Danny McGee