Make dock scrollable instead of extending off screen

[lenemter]

Fixes #485

[leolost2605]

This is really cool :)
The code LGTM :)

I think for now this makes sense to fix the issue and we can fine tune always later (maybe add an overflow menu instead of scrolling?) but I'll leave a ux review to @elementary/ux

[danirabbit]

This is cool! makes me wish we had a nice masked underscroll effect.

✅ I like the scrollbar being below the dock. Very cool. Looks good. Might need some custom CSS for the trough but no big deal
❔I think I like that there's separate scroll windows for launchers and spaces. Though I wonder if they should be equal sizes or if one should take priority

• I'd like to see the new workspace button be outside the scrollbar. I think that should be available always

[lenemter]

@danirabbit Done!

[lenemter]

With a single workspace, Gtk.ScrolledWindow reserves space for a scrollbar. We probably can put the scrolled into revealer and only reveal scrolled when there are workspaces. But this definitely needs some a bit more work