Uses the following shortcuts for the editor heading toolbar:
* Ctrl + Alt + 1: Heading 1
* Ctrl + Alt + 2: Heading 2
* Ctrl + Alt + 3: Heading 3
* Ctrl + Alt + 4: Heading 4
* Ctrl + Alt + 0: Paragraph
On macOS, substitute `Ctrl` with `Cmd` and `Alt` with `Option`.
Also adds composer shortcuts to keyboard shortcut help modal (?)