via marker improvements

[karussell]

This change implements four improvements for the via marker:

• drag from route to create a new via marker (the drag-start location determines the via number)
• click on route to create via marker (the click location determines the via number)
• delete via in ctx menu
• improves the via marker style and uses simple circles

Try it here: https://graphhopper.com/maps-dev/via_marker/

Drag from route works obviously only on desktop (hover) and looks like:

To make via markers more consistent with the transparent circle on that route I took the opportunity and changed the style of the via markers too (first commits). In master the via markers are a bit thin leading to rather small numbers. When changing this to a circle this looks cleaner, is consistent to the dragging style and the font can be also slightly bigger improving overall experience when using multiple markers.

Before:

After:

Now that the anchor is at the center of the icon the marker gets now transparent to achieve a better placement.