Enable Left / Right Segment Navigation also for projects with source editing and no alternates.

With TRANSLATE-118 we have introduced alternate targets and therefore Navigation Buttons (see screenshot) to jump between the alternates.

If a project do not have alternates, but source editing enabled this left / right buttons are currently disabled. The reason for this is the fact that change alikes are only usable by projects with no alternates (like the project mentioned above with source editing). In addition the left / right navi and change alikes are currently mutually exclusive in usage, so that we have to disable the buttons for projects with change alikes.

This should be changed, so that left / right navi is also available for non alternate projects with source editing enabled.