webadmin: Fix missing sub tab panel

In order for sub tab presenters and their parent presenters
to receive proper (consistent) life-cycle callbacks, we must
clear sub tab panel slot in MainContentPresenter when hiding
the sub tab panel from the view.

If we don't do the above, MainContentPresenter's sub tab panel
slot will still be occupied with obsolete sub tab presenter,
which will mess up sub tab presenter life-cycle callbacks,
such as onReveal not called each time when sub tab view is
shown in the view.

Change-Id: Ifc67def5083889deb360c9d48adf4d9e3b6786a9
Bug-Url: https://bugzilla.redhat.com/964949
Signed-off-by: Vojtech Szocs <vszocs@redhat.com>