Trace tool in edit

[steff-o]

Is your feature request related to a problem? Please describe.
When drawing new features it is hard to not create gaps or overlaps against other features in the same layer or other layers. Today it exists a snapping feature but if there are many vertices it is tedious work adding all vertices.

Describe the solution you'd like
Possibility to trace an existing feature by just following it.

Not sure how to activate it though. The obvious solution would be to activate trace using a modifier key (shift, ctrl, alt or whatever combination isn't already used), but that would not work well on mobile devices. Maybe not a problem as not many will do serious precision editing on a touch screen. Alternatively a new tool would be added to the create feature button. Feel free to discuss!

Describe alternatives you've considered
None, it's a non-negotiable end user request.

Additional context
The functionality exists in Openlayers as an option to the draw interaction. It does not exist on modify interaction, so it would only be implemented for new features. The same approach could be applied to the draw tools, but that is beyond the scope of this issue.

Demo (not mine): https://openlayers.org/en/latest/examples/tracing.html