Added animation target selector

wat-code-editor: need to expose the preview iframe so that the target selector can find all the DOM elements from the iframe.

timedItem-inspector: added an extra field to the inspector to show the animation target. Similar to the bezier editor, the target selector doesn't show up until the button is pressed. Necessary changes are made to hook up these two components.

wat-wat: we can't change the target of an animation directly and the work around is to create a new animation each time when the target is changed. If the animation being swapped out is the root, we need to make the root and the source of the current player points to the new animation.