Add some macOS emacs keybindings

[terhechte]

macOS has support for a subset of Emacs keybindings in all input fields. Here's an overview of these bindings / shortcuts. A input field that doesn't support at least a subset of this subset feels alien as it doesn't behave as expected. This PR implements some of those bindings:

• Move cursor left: ^B
• Move cursor right: ^F
• Beginning of line: ^A
• End of line: ^E
• Line up: ^P
• Line down: ^N

It doesn't implement move-by-word, selection or deletion, but I thought the above would be a good start.

[emilk]

I had no idea these key bindings existed on Mac :) Now I know