* They are now deinlined into source files, as most of the classes have
either heavy members (std::vector) or virtual methods.
* All of them are explicit now (that should be already done, don't know
why not).
* Passing huge classes by value and using move constructors to avoid
unnecessary copying.
In most cases the names aren't even supported/used and thus it is
wasteful to have them in all *Data classes. If the importer wants to
support them, it would reimplement *name() functions instead.
Some functions need to return "invalid ID", which in case of size_t
cannot be -1.
Also the plugins should behave the same in 32/64bit systems, so the type
should be the same for both.