Mark connectors

[arvind]

When creating a new mark (#229), we should be able to connect it to another such that the new mark's data and positioning is based off its host. There are two cases we would like to support:

• One new mark instance only (e.g., a single annotation).
• One new mark item/instance for every old mark item (e.g., labeling all points).

Besides the Lyra 1 method of dragging a new mark over an existing one to connect them, we should also explore Gregor Aisch's direct selection technique.