Draw letter and number markers with a special style

[herzbube]

Currently letter and number markers are drawn just like a regular label, although with a slightly larger font. Markers should instead be drawn with a noticeably different style to make them immediately recognizable as markers (e.g. circle around the digit/letter, different color, ...).

Whatever style is used, it must look OK both when the marker is on top of an empty intersection, and on top of a black or white stone.