Support SVG icons

[mprins]

Currently only bitmap / .png images are supported to be used as icons for the POI. See available icons:
https://github.com/mprins/dokuwiki-plugin-openlayersmap/tree/master/icons

expected behaviour

SVG icons should be rendered on the interactive as well as the static, printable map

ToDo

• Add SVG versions of available icons
• Add SVG rendering support to StaticMap
• ...