[Finishes #80768620] Allow blocks with different connects to be added to toolbox categories

Before: Blocks on the right can't be added to a category
<img src="https://cloud.githubusercontent.com/assets/413693/4666529/6ed55fae-5554-11e4-948e-290e34d3bd3d.png" width="400">

After: Categories are defined by listing blocks, not connecting them
<img src="https://cloud.githubusercontent.com/assets/413693/4666532/70585b4c-5554-11e4-9de5-1321e2c51946.png" width="300">

Out of scope for this change, but I wonder if in the future we may want to try to size this more dynamically - i.e. if we're loading toolbox blocks and see that we have a lot of blocks, make this height bigger.