Allow custom menu item for Drawer item --or-- support `badge` property

Context: defining a "Drawer item" in Designer.

Use case: I want to display a "badge" like the Tasks menu item:

Currently you can't specify your own element for the "DRAWER_ITEMS" slot contrib, it is hard-coded to `nuxeo-menu-icon`. But Designer does not expose the `badge` property either, so it's not possible
 to set this value.

So there are two possible improvements
:

• Expose the `badge` property
   in Designer - I'm not sure this is really possible though, because you need some code to set the value; maybe we could bind it to an automation call?

• Allow the user to specify
   the element to use for the menu item instead of hard-coding `nuxeo-menu-icon`.