Abstract diagrams: glue indications beyond names

[cgevans]

It would be helpful to have indications of glues on tile edges in abstract diagrams that are not just the glue names as text.

Considering various TAM diagrams, it would perhaps make sense to add color and marker properties for glues. The marker would use the the color. To avoid needing to keep track of tile and glue colors, it would make sense to have a black outline for the marker. Then behavior could be:

1. Without a color or marker set, no marker is displayed.
2. With just a color set, a default marker is displayed.
3. With just a marker set, only the black outline is displayed (fill is transparent).

Markers that might make sense:

1. Triangles (eg, review paper, or Tristan's diagrams).
2. Single/double lines (manually set, most likely): how would these look with outlines?
3. Small squares (also single/double)
4. Half-circles