Left arrow keyboard shortcut does not navigate up the header hierarchy

> An issue with the collapsible headings keyboard shortcut -- pressing 'left' correctly goes up to the parent header; however pressing it again should go to the grandparent header; it doesn't, instead going to the previous header of the same level.

Originally reported by @jph00 in #7041, as a regression from Notebook 6 to 7.

You can reproduce this by creating cells at the H1 and H2 levels:

1. Heading 1
a. Subheading 1a
b. Subheading 1b
2. Heading 2

When "Subheading 1b" is selected and not in edit mode, press LEFT. "Heading 1" should be active, not "Subheading 1a".