Take double-quote characters into account when parsing trigraph

For now, the function parse_trigraph() defined in gobject/glib-mkenums script was not taking double-quotes characters into account:

parse_trigraph('name="eek, a comma"') {'name': '"eek', 'a': None}

This patch take double-quotes characters into account:

parse_trigraph('name="eek, a comma"') {'name': 'eek, a comma'}

Closes issue #65 (closed)