Refactor Dialog abstraction (#90)

This PR overhauls the Dialog abstractions in `studio-ui` to be more
flexible for a wider variety of use cases, and adds a placeholder
"Import Asset" sub-dialog to the Media Library dialog.

framework/src/lib/deref.ts

@@ -0,0 +1,16 @@
+import { Ref } from "lit/directives/ref.js";
+
+export function deref<T>(refOrValue: Ref<T> | T | null | undefined): T | undefined {
+	if (isRef(refOrValue))
+		return refOrValue.value;
+
+	return (refOrValue ?? undefined);
+}
+
+function isRef<T>(refOrValue: Ref<T> | T): refOrValue is Ref<T> {
+	if (!refOrValue || typeof refOrValue !== "object")
+		return false;
+
+	const keys = Object.keys(refOrValue);
+	return keys.length === 1 && keys[0] === "value"
+}