This PR does some rudimentary cleanup / reorganizing to the
`studio-frontend` project.
### Changes
* Refactored `studio-frontend` to use the `<sts-canvas-provider/>` and
`<sts-viewer/>` components from `studio-web`
* Added some CSS layout utilities for positioning overlayed UI elements
* Extracted `<sts-toolbar/>` and `<sts-toolbar-group/>` to their own
components
* Extracted `<sts-transform-toolbar/>` to its own component