Cursor bullet is duplicated on two contexts that share a Lexeme

[raineorshine]

Steps to Reproduce

- a
  - cat
    - b
  - cats

1. Activate the context view on cat.
2. Set the cursor on either of the a contexts.

Current Behavior

The cursor bullet is rendered on both a contexts.

Expected Behavior

The cursor bullet should only be set on the selected context.

Notes

The cursor is identical in either case, since the a contexts are actually the same thought. However, cat and cats are different thoughts, and that information should be available in the LayoutTree. However, we may need to add a new state variable, cursorIndex, so that selectors in other components can access the context that is actually selected.