Stop updating the block workspace when it is hidden

[paulkaplan]

Because we are listening to workspace update events from the VM and updating the workspace directly in containers/blocks.jsx, updates that happen when the blocks are hidden (like adding a new sprite when on the costumes tab) are slow because they are updating the blocks toolbox/workspace.

We could save a lot of time by throwing those updates from workspaceUpdate to a reducer and then only update the workspace on blocks/componentDidReceiveProps when the blocks the same way we are updating the toolbox xml.