markers

Makes the openSelectedMarkers method protected instead of package
private.

Updated java doc with @since annotation.

Added a new test MarkerSupportViewTest

Change-Id: Ie428a5e1c81aa3db293fc62f7bf12f4426aa2e06
Signed-off-by: Enda O Brien <E.OBrien@pilz.ie>
Reviewed-on: https://git.eclipse.org/r/c/platform/eclipse.platform.ui/+/189994
Tested-by: Platform Bot <platform-bot@eclipse.org>
Reviewed-by: Jörg Kubitz <jkubitz-eclipse@gmx.de>