Add option to notify about version request

Cannot be configured for now.
Can be set via `adv.notify.discoversion` in the `notification` section.

Will notify about version requests via XEP-0092 and XEP-0232.

Client version can still be seen via caps (capabilities).
See `stanza_attach_caps()`.

src/config/preferences.h

@@ -57,6 +57,7 @@ typedef enum {
     PREF_FLASH,
     PREF_TRAY,
     PREF_TRAY_READ,
+    PREF_ADV_NOTIFY_DISCO_OR_VERSION,
     PREF_INTYPE,
     PREF_HISTORY,
     PREF_CARBONS,