Ignore xterm-DCS messages

Summary:
Would be ideal to process them too, but they seem kind of corner
case and ignoring them fixes vim having $q q written on the first
line on startup

BUGS: 383976

Test Plan:
Ran vim, looks good

Should this go to 17.08 or master?

Reviewers: hindenburg

Reviewed By: hindenburg

Subscribers: #konsole

Tags: #konsole

Differential Revision: https://phabricator.kde.org/D7644