The viewport collections panel show hierarchy now.
We add a new setting which let users set the hierarchy depth they
want to see of the collections in the viewport collections panel.
This way we can show them in an hierarchical yet controlled way.
The same setting and display order is used for the "Hide Collection" operator.
This is accessed with [alt]1-10 or Ctrl+H.
Note: I think this operator should move to bpy.ops.view3d (it is in
bpy.ops.object now). We are past API changes so I'm not touching this.