Workbench: Added the basics for the OverlayMode

Implemented the face orientation overlay for testing.
Overlay mode is only drawn when there are overlays to be rendered.
The overlay mode is rendered before the object mode.

release/scripts/startup/bl_ui/space_view3d.py

@@ -3473,6 +3473,7 @@ class VIEW3D_PT_view3d_display(Panel):
         col.prop(view, "show_outline_selected")
         col.prop(view, "show_all_objects_origin")
         col.prop(view, "show_relationship_lines")
+        col.prop(view, "show_face_orientation_overlay")
 
         col = layout.column()
         col.active = display_all