ubuntumirclient: Stick-to-the-finger smooth scrolling

Similar to other platforms, this implements a basic version of Qt's scroll phase mechanism to tell the application specifics about when a mouse wheel or trackpad scroll event has occurred, from beginning to end.

Delivery is delayed by one event to keep the tracking simple and cover the most important phases for proper scrolling in QML. One timer is responsible for initiating the momentum phase whereas another one completely ends the touch input altogether.

This change makes QtUbuntu depend on Qt 5.12 due to the use of the momentum phase, 5.12 being the version it was introduced.