wip: use CircleMarker instead of Circle icon

[yohanboniface]

This needs more work, but would allow:

• better performance: - one svg for all features instead of one HTML per feature - as it inherits from path, only visible feature are added to DOM
• controling the circle radius, which is something requested from time to time

To move forward, we need:

• to have default per UI class (instead of in the SCHEMA), as for example we do not want the same default weight for a line or for the CircleMarker
• maybe to have some static properties, so to keep the circle border color always white, as now (but can be discussed)
• when editing a feature, to show properties according to the current selected UI class, so user can define the radius when "Circle" is selected, but also other path related properties (fillColor, opacity…)