New python method gdb.Objfile.add_separate_debug_file.

New python method gdb.Objfile.add_separate_debug_file.

gdb/ChangeLog:

	* NEWS: Mention gdb.Objfile.add_separate_debug_file.
	* python/py-objfile.c (objfpy_add_separate_debug_file): New function.
	(objfile_getset): Add "add_separate_debug_file".

gdb/doc/ChangeLog:

	* python.texi (Objfiles In Python): Document
	Objfile.add_separate_debug_file.

gdb/testsuite/ChangeLog:

	* gdb.python/py-objfile.exp: Add tests for
	objfile.add_separate_debug_file.