Add warning when pasting unprintable chars and ask to filter them out

Summary:
inspired by https://georg.so/pub/cat.html and other semi-recent
stuff, having a more general safety mechanism seemed like a
good idea.

Test Plan:
https://georg.so/pub/cat.html is a good start,
https://github.com/minimaxir/big-list-of-naughty-strings/blob/master/blns.txt
has some as well

Reviewers: #konsole, hindenburg

Reviewed By: #konsole, hindenburg

Subscribers: hindenburg, konsole-devel, #konsole

Tags: #konsole

Differential Revision: https://phabricator.kde.org/D17293