Room background settings are stored in the level data, should only store the background type

[city41]

Currently the level format stores bgGraphic, bgColor and bgExtraColorAndEffect as individual values in the level data. The intent was to allow people to customize these values. The downside is whenever one of these values proves to be wrong, the only way a level can get fixed is for the user to change the background to something else then back again.

tasks

• only store the background type (ie plains, fortress, etc) in the level data
• in createLevelData, look up the current values for bgGraphic,color,extraColor based on the type
• add a version migration to move all current levels to this
• think about allowing overrides for advanced usages in the future.