webadmin: User Event Notification dialog element IDs

1. UI infrastructure changes

   - created ElementIdCellTree that supports assigning DOM
     element IDs to tree nodes through HasElementId interface

   - EntityModelCellTree derives from ElementIdCellTree,
     so all model-bound trees now support node element IDs

   - ModelListTreeViewModel does the actual element ID
     assignment for the CheckboxCell

   - example tree node ID, e.g. 2nd child of 3rd root node:
     <prefix>_root2_node1

2. WebAdmin dialog changes

   - User Event Notification dialog (ManageEventsPopupView)
     now supports element IDs for Tree and Expand/Collapse
     buttons

Change-Id: Icf18b0080fffe6ba3886736a605f53570889bf3a
Bug-Url: https://bugzilla.redhat.com/870993
Signed-off-by: Vojtech Szocs <vszocs@redhat.com>