Add auto-scroll functionality when selecting.

[anmcgrath]

Adds auto-scroll when suggesting, as suggested in #229.

I tried to emulate excel's behaviour somewhat, the scrolling speed increases as the mouse is further away from the edge of the scroll container.

[kei-soft]

Thanks for reflecting my request so well. I really appreciate it!

[anmcgrath]

Thanks @kei-soft I want to improve the auto-scroll when selecting cells from the formula editor, as well as make some improvements to scrolling when selecting regions with the keyboard before I finish this PR.