LGTM.com-inspired changes:

- Fix: Ensure all apostrophes are escaped for `toXml` utility
- Fix: Avoid error if `URL` is not defined
- Fix (jPicker): Precision argument had not been passed in previously
- Fix (Star extension): Minor: Avoid erring if `inradius` is `NaN`
- Refactoring: Avoid passing unused arguments, setting unused variables,
  and making unnecessary checks; avoid useless call to `createSVGMatrix`
- Linting (LGTM): Add `lgtm.yml` file (still some remaining items flagged
  but hoping for in-code flagging)
- Docs: Contributing file

editor/extensions/ext-markers.js

@@ -279,7 +279,7 @@ export default {
       const poslist = {start_marker: 'start', mid_marker: 'mid', end_marker: 'end'};
       const pos = poslist[this.id];
       const markerName = 'marker-' + pos;
-      let el = selElems[0];
+      const el = selElems[0];
       const marker = getLinked(el, markerName);
       if (marker) { $(marker).remove(); }
       el.removeAttribute(markerName);
@@ -294,7 +294,9 @@ export default {
       const id = markerPrefix + pos + '_' + el.id;
       addMarker(id, val);
       svgCanvas.changeSelectedAttribute(markerName, 'url(#' + id + ')');
-      if (el.tagName === 'line' && pos === 'mid') { el = convertline(el); }
+      if (el.tagName === 'line' && pos === 'mid') {
+        convertline(el);
+      }
       svgCanvas.call('changed', selElems);
       setIcon(pos, val);
     }