MACRO: add speicific macro for decorating interfaces

A set of new macro is introduced designed to declare and implement
interface classes, inherited from objbase and have SINGLE realization
in obe derived class (very common situtation in BS Eagle plugins). Such
interfaces are used to skip explicit linkning between different plugins
when using MSVS compiler.
readme
misc: string <-> wstring converters using deduced native system encodind
misc.python: use native encoding for auto wstring -> string conversion from Python