webadmin: Prevent SystemTree from losing selection

There are basically two listeners for SystemTreeModel's
ItemsChanged event:

  (1) SystemTreeModelProvider.updateDataProvider
      - actual listener registration code is in
        DataBoundTabModelProvider.onCommonModelChange
      - this method selects root "System" node

  (2) SystemTree.addModelListeners
      - this method de-selects last selected node

Upon WebAdmin startup, (2) gets executed after (1) which
causes SystemTree to lose selection.

As a result, code calling SystemTreeModel.getSelectedItem
might blow up on NullPointerException.

This patch ensures SystemTree doesn't lose selection upon
SystemTreeModel's ItemsChanged event.

Change-Id: Ie5c099dd6bbce0f721e7ed2430ecb7124b9241ca
Signed-off-by: Vojtech Szocs <vszocs@redhat.com>