webadmin: UI plugin event handler

- Changed the way UI plugin events are handled so they don't
  instantiate all the presenters at once.

Change-Id: I83ad213a1dca8d31cf63c6b2c5f761f3242b43bc
Bug-Url: https://bugzilla.redhat.com/show_bug.cgi?id=1155992
Bug-Url: https://bugzilla.redhat.com/show_bug.cgi?id=1148095
Signed-off-by: Alexander Wels <awels@redhat.com>