make Feature.hasRequiredAttributes protected to allow reuse by
extensions

Change-Id: I39d6cdddc3d3c0e233f3a705264f862949930da8
Signed-off-by: Christoph Läubrich <laeubi@laeubi-soft.de>
Reviewed-on: https://git.eclipse.org/r/c/pde/eclipse.pde.ui/+/186434
Tested-by: PDE Bot <pde-bot@eclipse.org>
Tested-by: Vikas Chandra <Vikas.Chandra@in.ibm.com>
Reviewed-by: Vikas Chandra <Vikas.Chandra@in.ibm.com>
Reviewed-by: Alexander Fedorov <alexander.fedorov@arsysop.ru>