Measuring angles

Proposal from @aberenyi
Allow independent customization of heading labels.

And:

* Prepare the way to customize the position of the labels.
* Adjust callbackUndo to also work with layerHeading.
* Simplify the code in some cases.

TODO: customizable position, docs, examples, changelog...
Allow customize label positions.
By default does not show hading.
Activate/deactivate heading on example.
Remove duplicate keys and fix other unimportant details.
Update docs