Secondary identifier "squiggle"

[allantokuda]

This is lower priority than most of the other LDS features.

In addition to identifier bars, there can be identifier "squiggles". Anything that can currently have a bar (attributes and links) should also be able to have a squiggle, or both a bar and a squiggle at the same time.

This will need its own editor mode. Can be invoked with keypress ~. This is not an often-used feature, so we will want to hide it away. In the toolbar we can have a down-arrow that expands to a menu of additional features, and this can be the first item in the list.

In terms of SVG, this basically doubles the number of arrowheads. And I think drawing this inside the entity will require adding one or more SVG elements inside the entity div element.