Fixes#177
Render navigation component arrows for sequence as well as
spatial.
Position arrows according to direction and sequence component.
Use base64 encoded svgs rotated with svg transform to avoid
more svgs in dist folder.
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).
