textinput: introduce a signal for when the state should be committed

In v1 to v3 of textinput protocol, updates for the text input state
needs to be updated explcitly using commit or update state request, so
send out this information to compositor, which can use it to update the
input method state.

src/wayland/server/textinput_interface.h

@@ -396,6 +396,11 @@ Q_SIGNALS:
      * @see surface
      **/
     void enabledChanged();
+    /**
+     * Emitted whenever TextInputInterface should commit the current state.
+     * @see requestReset
+     **/
+    void stateCommitted(uint32_t serial);
 
 protected:
     class Private;

src/wayland/server/textinput_interface_v2.cpp

@@ -224,11 +224,11 @@ void TextInputUnstableV2Interface::Private::updateStateCallback(wl_client *clien
 {
     auto p = cast<Private>(resource);
     Q_ASSERT(*p->client == client);
-    Q_UNUSED(serial)
     // TODO: use other reason values reason
     if (reason == ZWP_TEXT_INPUT_V2_UPDATE_STATE_RESET) {
         emit p->q_func()->requestReset();
     }
+    emit p->q_func()->stateCommitted(serial);
 }
 
 TextInputInterface::ContentHints TextInputUnstableV2Interface::Private::convertContentHint(uint32_t hint) const