qml: filter rapid events on position change in seekbar

Qt has built-in event compression for certain events, such as window resize. However, this is not the case with mouse move event. In such cases, filtering needs to be handled manually.

This approach is already followed in the volume slider. It is not a good idea to request backend to do an operation just to override it within the same event loop processing cycle. Without filtering, CPU utilization increases a lot and stuttering may be observed.

Volume or player position update must be asked to the backend at most once per Qt event loop cycle. With Qt.callLater() this can be satisfied.