LuaRef & PriorityMap serialization

This patchset adds support for LuaRef and PriorityMap serialization. I haven't enabled the priority serialization since it would require a save bump, and I wanted to collect opinions on that.

On one hand, it's always a PITA to make all savegames obsolete, but on the other hand this would allow for modders to actually use the properties. Right now, to use them one has to reapply all changes manually when loading, which isn't very nice.

Please note that I'll enable the property saving in new-equipment regardless, as I for one really don't want to reapply all equipment modifiers each time I load/save (and I'll have to bump the savegame version anyway)