webadmin,userportal: Small ScrollableTabBar improvements

1. ScrollableTabBarView:
   - removed "implements HasLoadHandlers"
   - removed fireEvent + addLoadHandler methods
   - the "+1 width adjustment" comment moved to
     appropriate location

2. ModelBoundTab:
   - added setAccessible override that fires
     TabAccessibleChangeEvent whenever tab's
     accessibility changes

Change-Id: Ib7ffdb2827067b35320fd43d37db80479df6605d
Signed-off-by: Vojtech Szocs <vszocs@redhat.com>