Feature: setting to indicate snow coverage for arctic climate (replaces snow line height)

Setting the snow coverage (in % of the map) makes a lot more sense
to the human, while still allowing the niche player to set (by
finding the correct %) a snow line height they like. This makes for
easier defaults, as it decoupled terrain height from amount of snow.

Maps can never be 100% snow, as we do not have sprites for coastal
tiles.

Internally, this calculates the best snow line height to approach
this coverage as close as possible.

src/settings.cpp

@@ -826,6 +826,7 @@ bool SettingDesc::IsEditable(bool do_command) const
 	if ((this->desc.flags & SGF_NEWGAME_ONLY) &&
 			(_game_mode == GM_NORMAL ||
 			(_game_mode == GM_EDITOR && !(this->desc.flags & SGF_SCENEDIT_TOO)))) return false;
+	if ((this->desc.flags & SGF_SCENEDIT_ONLY) && _game_mode != GM_EDITOR) return false;
 	return true;
 }