screen write protection

[tehn]

currently it's common for people to get in situations where the screen is written to in various parts of the code outside of redraw which will bust the menu code in various ways.

the screen lib and menu handling should deal with this more thoroughly, so there aren't so many "don't do this" caveats

side issue is managing secondary screen-stealers within scripts like fileselect/etc (this part is tricky)