Fix msgctxt support for Glade2 files.

xgettext-glade-5: New test for glade2 msgctxt extraction.

xgettext.h:
    split_glib_syntax_string: New global function.
xgettext.c:
    split_glib_syntax_string: Implementation from arglist_parser_done.
    arglist_parser_done: Substitute the old code.
    			 Fix a couple of memory leaks on best_cp->msgid
			 and best_cp->msgid_plural when a msgctxt has
			 been extracted.
x-glade.c:
    element_state: New field extract_context.
    start_element_handler: Set p->extract_context.
    end_element_handler: Implementation of msgctxt extraction using
                         split_glib_syntax_string.
Bug fixes from Savannah bug #36063.

'@opindex --lang@r{, @code{msgcat} option} in msgmerge.texi and msgen.texi.
Merge branch 'proposed_changes' into glade_bugs

README modified.