Promote / Demote Heading TreeItem

[dwarry]

Allow the UI to promote/demote Headings in the Page Content Mind Map.

Promoting an item moves it from it's current parent to the grandparent's Children collection.
Headings at Level 1 cannot be promoted.

Demoting an item moves it to the end of the previous sibling's Children collection; the zeroth item of a particular batch cannot therefore be demoted as there is no prior sibling.

Headings at level 6 cannot be demoted.