It prevents unwanted implicit conversions from e.g. nullptr to Camera,
Vector2 to Physics::Point etc. By making all the constructors explicit
it is easier to routinely add the keyword to all new classes instead of
thinking about cases when to add and when not to.
Extern template probably causes even inline functions to be
instantiated, because MinGW's GCC 4.7.0 then complains about conflicting
symbols, removing them fixes the issue.
Extern template is not necessary here, as the needed functions are
explicitly instantiated in source file only anyway and we don't care
about instantiation count of inline functions.