Key codes indistinguishable from pasted text

Because the codes for keys that do not directly have a character assigned to them are strings, they don't can not be distinguished from valid text.

This could be solved by adding a 'Key' type, and make all keycode constants of that type, instead of strings.
Instances type would only need one property: the name, which is the same the current string vaue

Start by locating the keycode constants and the code that currently represents non-character keys as strings. Define what the proposed Key type must distinguish from valid pasted text, then trace all uses of those constants to determine the affected behavior and verification needed.

Written by the indexing model from the issue text.