Add a scroll bar after zoom in timeline

[userzzzq]

that will be better

[markusd1984]

sweet, never occurred to me that one can zoom in via mouse wheel scroll :D despite me using it regularly in premiere pro :)
There is a nice empty space in the right bottom corner underneath the timeline where a zoom slider could fit perfectly.

[mifi]

we already have a zoom selector. I'm not sure whether select or slider is the best. sliders can also become quite fiddly when small

[mifi]

closing in favor of #483