Enable cycling header levels with ctrl+h.

Addresses #3469.
Addresses #3019.

* Remove preexisting `ctrl+h` keymap.
* Add logic to cycle the header level.
  * Loops around after h6.
  * Maintains cursor position.
  * Set initial level to h2.
* Remove `ctrl+alt+${NUM}` keymaps (as they do not work
  internationally).
hrm, maybe clump this in with Text now that there's only one heading shortcut?