Only update the toolbox when the blocks tab is visible.

Need to store the rendered xml because the props can change while the blocks tab is hidden, but we re-render based on what was rendered, not the previous props (which may not have been rendered).

This is similar to the `ThrottledPropertyHOC` in that there may be prop changes that do not get rendered, but in this case for a different reason.

This increases the performance of switching between code and other editor tabs, as well as improving the "See Inside" performance. It makes switching to the code tab as fast as switching between sprites.