Enable cycling header levels with ctrl+h.

Note: This is a cleaned up version of #3484 - I've rebased it and modified the behaviour to cycle through h2 and h3, otherwise it's the same PR & it's @rwjblue's commit ;)

* 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).