style with symbols

[Nicolasribot]

A cleaning of a work started some years ago to support a style displaying symbols. Symbol icons are based on OSM icons converted from SVG. This style can be combined with other styles to add icons. WIP: labels at z18, more properties should be defined in generate_styles.py to control symbols behaviour. Some symbols are missing (parking built from ways for instance). Some examples of the style: https://github.com/Nicolasribot/basemaps/blob/master/README.md

[yjacolin]

@Nicolasribot thank you for you huge work on this. Unfortunatly this PR is too big and we can review it correctly. Could you split it in few part? Scripts, topobase and symbols at least? My browser failed to display diff file after "scripts".

[Nicolasribot]

Yes I will make the changes.