Allow setting stroke-dasharray on lines

[dkleszyk]

It would be nice to be able to style lines as dashed or dotted in addition to setting their color.

Adding some way to set the stroke-dasharray attribute on paths should accomplish this.

It's probably only needed on the foreground line; the background line used for contrast can stay solid.

[cdauth]

I have implemented a “Stroke” setting with 3 options (solid, dashed, dotted) for lines.

The setting can already be tested on https://beta.facilmap.org/ (note that FacilMap Beta contains a copy of all maps, but any modification made there will be lost once the next release is made).