Planet data: More flexibility

While writing the LUA “planetTick” function, I came to the conclusion that requiring modification of the C++ core to add new kinds of data to planets runs counter to the premise of user modifiability.  So I’ve added an “extra data” child to the planet XML node, which can contain arbitrary keys and values, which are made available to LUA through the getPlanetData function.  New data can be added either through editing the XML or in LUA through the setPlanetData function, and all of this will be stored on save.