window-padding-color improvements

[mitchellh]

• Add heuristic so power line cells disable extend
• Add extend-always configuration
• Use exhaustive switch statements so future values could be caught
• Don't always extend in alt screen, this was very uniformly disliked except for @qwerasd205 (you can now use the new extend-always which isn't identical but probably good enough)
• This makes extend the default again. I think the heuristics in this PR plus previous ones address all the major "ugly" scenarios to make this worth it.

cc @qwerasd205