Allow unlinking Library symbols

Sometimes importing large libraries is not required/practical (e.g. Large libraries)

In this case user should be give a choice to export the library-symbols unlinked. 

• Preferably the library-symbol should be made a document symbol and exported as document-symbol. If this is technically not possible, the library-symbols should be unlinked and exported.
• Do deep unlinking. The library-master can depend on other libraries.  

• This process should not mutate the document. The library-symbols are mutated temporarily.