Make QtFeedback dependency optional

Hi! I've faced similar issue with Plasma Dialer: plasma-mobile/plasma-dialer#51 (closed)

Is it makes sense to make QtFeedback dependency optional to simplify building for the distros without qt feedback support via the repos (due to missing maintenance). For the dialer case it is handy for the CI/CD task with the openSUSE Tumbleweed images without QtFeedback: https://phabricator.kde.org/T15755

I've found implementation approach from maliit keyboard, I could introduce something similar: https://github.com/maliit/keyboard/commit/2220e3e5c94c3b218157c7b40e6b77d003a78181

Feel free to close, maybe not relevant or some other approach is going to be used in the future.