UI: enhance "Window" menu
UI: enhance "Window" menu

Add a separator before adding entries to not confuse HeeksCAD toolbars and plugins adds
elliotf/heekscnc,ant-t/heekscnc,blakelyc/heekscnc,hackendless/heekscnc,dnevels/heekscnc,hackendless/heekscnc,elliotf/heekscnc,BbiKkuMi/heekscnc,elliotf/heekscnc,AlanZheng/heekscnc,dnevels/heekscnc,JamieFBousfield/heekscnc,ant-t/heekscnc,dnevels/heekscnc,blakelyc/heekscnc,AlanZheng/heekscnc,BbiKkuMi/heekscnc,JamieFBousfield/heekscnc,masika/heekscnc,FluffyMortain/heekscnc,FluffyMortain/heekscnc,dnevels/heekscnc,ant-t/heekscnc,Nurb432/heekscnc,FluffyMortain/heekscnc,JamieFBousfield/heekscnc,AlanZheng/heekscnc,elliotf/heekscnc,singwina/heekscnc,pyrotron/heekscnc,Nurb432/heekscnc,BbiKkuMi/heekscnc,blakelyc/heekscnc,sangit/heekscnc,masika/heekscnc,singwina/heekscnc,sangit/heekscnc,singwina/heekscnc,JamieFBousfield/heekscnc,ant-t/heekscnc,pyrotron/heekscnc,sangit/heekscnc,AlanZheng/heekscnc,blakelyc/heekscnc,sangit/heekscnc,hackendless/heekscnc,pyrotron/heekscnc,Nurb432/heekscnc,hackendless/heekscnc,BbiKkuMi/heekscnc,pyrotron/heekscnc,Nurb432/heekscnc,masika/heekscnc,masika/heekscnc,FluffyMortain/heekscnc,singwina/heekscnc