Interactive (e.g. navigation, attribution) element have z-index 10.
Overlayed elements (slider, route bubble) have z-index 20.
Filling elements have z-index -1 to 9.
Cover have z-index 100.
Special elements (debug) have z-index 50.
Do not change z-index in server cssnano plugin (z-index are not
changed when built from npm script).
Use crosshair when creating tags.
Use crosshair for editing vertices.
Use pointer for icon.
No pointer events on container when not creating to be able to
fall back to other element cursors.
Add support for creating new polygons.
Remove last vertex by clicking it.
Abort creation by clicking original vertext when fewer than 4.
Complete polygon by clicking original vertex.
