Add language to longname in --xml output

Without that, many ibus-m17n engine become indistinguishable in the engine switcher shown by Super+Space because they have exactly the same icons.

For example /usr/share/m17n/icons/hi-inscript2.png and
/usr/share/m17n/sa-itrans.mim are identical. Adding the language to
longname makes it possible to see the difference in the engine
switcher.

src/m17nutil.c

@@ -125,7 +125,7 @@ ibus_m17n_engine_new (MSymbol  lang,
 
     engine_name = g_strdup_printf ("m17n:%s:%s", msymbol_name (lang), msymbol_name (name));
 
-    engine_longname = g_strdup_printf ("%s (m17n)", msymbol_name (name));
+    engine_longname = g_strdup_printf ("%s-%s (m17n)", msymbol_name (lang), msymbol_name (name));
     engine_title = ibus_m17n_mtext_to_utf8 (title);
     engine_icon = ibus_m17n_mtext_to_utf8 (icon);
     engine_desc = ibus_m17n_mtext_to_utf8 (desc);