Categorize does not work on multiselected thoughts in nested context view

[raineorshine]

Steps to Reproduce

- a
  - m
    - x
      - f
      - g
- b
  - m
    - y

1. Set the cursor on a/m.
2. Activate Context View.
3. Set the cursor on a/m~/a/x/f.
4. Activate Select All to add a/m~/a/x/f and a/m~/a/x/g to the multicursor.
5. Activate Categorize.

Current Behavior

Nothing happens.

Expected Behavior

The selected thoughts should be categorized with a new empty thought, just like in normal
view.

The context view should stay open.

- a
  - m
    - x
      - [empty thought]
        - f
        - g
- b
  - m
    - y

Testing

Add appropriate unit test.