BUGFIX: Handle text input in keypress handler for I18n

While the `keydown` handler is the only way of reliably catching special keys,
for textual input `keypress` is preferable, as it can handle thing like parens
without having to hardcode a keyboard.

I moved that code and in the process converted some magic numbers to constants
- mostly so I can read stuff better.
Merge pull request #2833 from mamhoff/new-bugfix-autocomplete

BUGFIX: Handle text input in keypress handler for I18n