More markdown shortcuts

## Problem

Code only has very basic markdown shortcuts.

## Proposal

* Make the backticks \` work like parenthesis: adding a single backtick adds a double and selecting text and inserting a backtick surrounds the text with it. Backtick is used in markdown to create `monospaced text`
* Use `Ctrl`+`Shift`+`1` up to `Ctrl`+`Shift`+`6` for headers. Make it a toggle shortcut.
* `Ctrl`+`Shift`+`P` for inserting images (**p**ictures, get it? ;). Maybe even use a file chooser to select the file.

No files, tests, or entry points are named. Start by locating the editor's markdown shortcut and keybinding handling, then determine how image insertion is implemented; done means backticks, heading toggles, and image insertion work as proposed without regressing existing shortcuts.

Written by the indexing model from the issue text.