gdb/python: new Progspace.symbol_file attribute

Add a new Progspace.symbol_file attribute.  This attribute holds the
gdb.Objfile object that corresponds to Progspace.filename, or None if
there is no main symbol file currently set.

Currently, to get this gdb.Objfile, a user would need to use
Progspace.objfiles, and then search for the objfile with a name that
matches Progspace.filename -- which should work just fine, but having
direct access seems a little nicer.

Reviewed-By: Eli Zaretskii <eliz@gnu.org>
Approved-By: Tom Tromey <tom@tromey.com>