editor grid & clue: show cursors

[stevenhao]

show cursors of collaborators, like in games.
this will be trickier, because it matters (moreso) what direction the cursors are associated with.
it also may be nice to know, e.g., if your collaborator is editing a clue / the position within the clue.

Checklist

• grid cursors (0fe7989)
• grid cursor orientation
• cluetext cursors
  example composition page: https://downforacross.com/composition/263596