[Finishes #69033760] Support toolbar with concreteBlocks

- Give blocks/svgBlocks the ability to set visibility (using style:display).
- Create concreteBlocks option
- When dragging an item out of the toolbox, if concreteBlocks is true, we hide the block in the toolbox
- When a block is dragged into the toolbox, we look for a hidden block of this type, and unhide it.