widl: Consolidate writing of COM and dispatch interfaces into one function to remove duplicated code.

Split up the writing into start and end to eventually support the style 
MIDL uses where it writes declared types, etc. between the start and end 
of the interface.

Make internal header functions take the file pointer to print to.

Don't write interface IDs for non-object interfaces and always write 
handle declarations even if the interface has no methods, like MIDL does.