Feature request: Support css classnames for <category> blocks

Problem statement

Currently it is not possible to set individual css classnames for the different <category> blocks which limits the current styling options.

Expected Behavior

It should be possible to pass the css class names like:

<category name="..." colour="120" class="some_css_class some_other_css_class">
  ...
</category>

Which should be translated to:

<div class="blocklyTreeRowItem some_css_class some_other_css_class" id=":w" role="treeitem" aria-selected="false" aria-level="1" aria-labelledby=":w.label">
 ...
</div>

Additional Information

I currently solving this by using an loop and adding css classnames dynamically over javascript.

[alschmiedt]

You can now add a css class to a specific category.

[MarkusBordihn]

At https://github.com/google/coding-with-chrome/blob/master/src/ui/blockly/blockly_toolbox.js#L79 you can see how this implementation is done on my side. Please keep in mind that I’m not using the latest version yet, so it could be you need to adjust this example to get it work.