Use libfeedback

This switches from gsound to libfeedback to provide haptic feedback. Current downsides are

• libfeedback does not care about the sound theme so we get the default ringtone (feedbackd#17)
• libfeedback is not as precise as gsound atm when ending feedback (that does not matter much with the current ringtone though) (feedbackd#10 (closed))

I've left the notification part in this MR as well for the moment.

Closes: #130 (closed)