[WIP] Add export of KML

This is work in progress. Status on FF/Pale Moon
### Export from Drawing dialog
- [x] Points (including sizes)
- [x] Polylines (including coloring)
- [x] Polygons (including coloring)
- [ ] Text -> does not work

### Export from measure dialog
- [ ] All
I'm pretty sure, someone will ask us to export a GPX file in a near future. We could have a generic gaExport service and pass the format as an option, what do you think?