Single-canvas juggling (#15)

This PR adds a `CanvasProviderElement` to `@storyteller/studio-web` to manage
the complexity of maintaining a single concrete `HTMLCanvasElement` for
`@storyteller/studio`, while still being able to treat components like the
`ViewerElement` as self-contained "views" that behave as expected with regards
to CSS styling, DOM placement, etc.

Consuming applications should wrap the new `<sts-canvas-provider />` element
around some common ancestor of all views into the `@storyteller/studio` app,
similar to a React context provider.

The `react-frontend` application has been updated to take advantage of the new
`CanvasProviderElement` by demonstrating a more realistic use case for the
`ViewerElement`. The new demo presents a grid of "thumbnails," one for each
object in our hard-coded "object library." Clicking on one of these thumbnails
will render a `ViewerElement` for that object in a modal dialog.

react-frontend/src/app/studio-canvas-provider.tsx

@@ -0,0 +1,15 @@
+import { CanvasProviderElement } from "@storyteller/studio-web/canvas-provider";
+import "@storyteller/studio-web/canvas-provider";
+import React, { ReactNode, forwardRef } from "react";
+
+export interface Props extends React.HTMLProps<CanvasProviderElement> {
+	children?: ReactNode | ReactNode[];
+}
+
+export default forwardRef<CanvasProviderElement, Props>(
+	({ children, ...props }, ref) => (
+		<sts-canvas-provider {...props} ref={ref}>
+			{children}
+		</sts-canvas-provider>
+	)
+);