Support functional keys

[jespertheend]

I think it could be useful to be able to get the values for keys such as AltLeft and Escape etc.

In general, macOS users expect shortcut hints to be formatted as '⇧ ⌘ N', whereas windows users would expect something like 'Ctrl + Shift + N'. Having the map contain values for functional keys would make it easier to communicate shortcuts to the user across different platforms.