Implement simple easing curve editor for timeline

The timeline is keyframe based. Easing curves describe the interpolation of values between two consecutive keyframes.

We need a simple easy to use editor for easing curves and a way to select provided easing curves. We looked at existing tools and I provided a working QML prototype that can be turned into a full-fledged implementation.

Eventually, we will add a more complex easing curve editor that allows more than one segment. This will essentially be the polished tool from qtdeclarative.