Timeline Docker Redux: Scrollbar Realignment and Zoom Feature

This is a commit toward the larger project: Animation Timeline Redux.

This patch gives the timeline docker the ability to zoom while scrolling by moving the mouse up and down while holding onto the scrollbar handle. It also improves some behavior regarding the infinite scroll by adding an overscroll signal and overriding some default scrollbar input handling. It also realigns the scrollbars to account for the header offsets. Lastly, it improves the infinite frame table by ensuring that the entire playback range is accessible.

This can eventually be reused in the curves docker too, when I get to it.

I would like an appimage to give to artists for testing purposes @dkazakov if you wouldn't mind. For future reference, if I want to make an appimage, do you recommend using the docker container to do so?