pohelper: Initial revision
pohelper: Implement message fuzziness toggling
pohelper: Add keybindings to go to any previous/next message

Add keybindings for previous and next message, no matter whether they
are translated or not.
pohelper: Properly escape inserted name and email

Make sure inserted name and email don't contain control characters
such as quotes (") or backslashes (\).
pohelper: Fix reflow to use the number of characters, not bytes
pohelper: Add a menu under Tools
pohelper: Add a setting to choose whether to update headers on save