Fix display sometimes randomly scrolling down

Something in Qt has changed, so e. g. Super_L isn't passed as a modifier
anymore, so there's a regression from what
eea5ecfc5ed93515afaae5b9b938154eeaaf0ba8 fixed.

This is especially annoying e. g. for people using i3, where just
switching focus away with the keyboard will lead to Konsole scrolling
back to the bottom.