Paginate/scroll the site list

[narc0tiq]

By request from @cpw (and also from common sense): when the site list gets too long, it can go off the bottom of the screen. The number of sites per page should be configurable within reason (e.g., limit to a range of 10-25).

[narc0tiq]

Factorio 0.17.35 just fixed this for me, technically:

Mod GUI root elements (top/left/center/goal) are now contained in scroll panes, so when there is not enough space, it will become scrollable instead of being cut off.

Of course, now I'm thinking I should still make the site list live in a scroll pane, because that would keep the filter buttons from getting scrolled along with the rest of the list. Needs testing, but much lower priority now.