[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">

looks like Nokogiri adds the missing closing bracket when parsing in any case, but it makes no difference:
```ruby
> Nokogiri::XML("<test").to_xml
"<?xml version=\"1.0\"?>\n<test/>\n"

> Nokogiri::XML("<test/>").to_xml
"<?xml version=\"1.0\"?>\n<test/>\n"
```
but this was probably a typo in the first place rather than something intentional.