Reintroduce stub code from XPIDL generated header files

https://bugzilla.mozilla.org/show_bug.cgi?id=1386962

Part of the point of this project is to enable custom compiled XPCOM components. Those templates were really useful when generating new C++ code.

That said, I don't plan on adding this back to the Mozilla code. Instead, I'd adapt the removed code into a new Python terminal application which would generate the stub and place it in a user-specified source directory.