Changes

=== Trailer def ===

Structure '''trailer_def''' was greatly improved and redesigned as its core of the economy. Old system with list of trailers for given cargo is used for rest of old trailers only (using cargo folder placement with filled '''trailer''' and fake underscored trailer body types).

New fields support system that matches cargoes to trailer only through trailer body types with additional information. Most important are '''gross_trailer_weight_limit''', '''body_type''' and '''country_validity'''. Then there are cached/precomputed '''trailer_mass''', '''body_mass''', '''volume''', '''axles''', '''length''' and '''chain_type''' for fast access without loading all accessories as well as data for trailers that do not have all accessories well defined (eg freight market ones). In adition, there are also cog placement parameters - unlike older mechanics right now its plain cargo cog offset in meters (before was height in halfs due to historical reasons).