Mark boxes not drawn solid

[westram]

Traditional display showed empty groups and solid mark boxes. Now both are always drawn with the same grey level.

Non-solid mark boxes are ugly: the branch line continues to the center of the box.

Possible solutions:

1. always draw mark boxes solid (recommended)
   • change bool filled param into enum {FILL_NONE, FILL_GREY, FILL_SOLID };
2. allow grey level for mark-boxes
   • add separate grey level setting for mark boxes (default to "solid" to preserve old behavior)
   • change draw functions for all -types to draw shorter lines

[westram]

fixed somewhere together with marker display (added with probe spec)

[westram]

remove gtkport tickets from milestones