[kwin/kcmdeco] Show an information if close window by dbl click gets activated

The option changes the behavior of the menu button, thus we should point
out to the user that the behavior changes.

This is only done for Auroae configurations as other decorations have to
take care about it themselves.

CCBUG: 331462
REVIEW: 116715

kcmkwin/kwindecoration/auroraeconfig.ui

@@ -11,6 +11,10 @@
    </rect>
   </property>
   <layout class="QVBoxLayout" name="verticalLayout">
+   <item>
+    <widget class="KMessageWidget" name="doubleClickMessage">
+    </widget>
+   </item>
    <item>
     <layout class="QFormLayout" name="formLayout">
      <item row="0" column="0">
@@ -131,6 +135,12 @@
    <extends>QComboBox</extends>
    <header>kcombobox.h</header>
   </customwidget>
+  <customwidget>
+   <class>KMessageWidget</class>
+   <extends>QFrame</extends>
+   <header location="global">kmessagewidget.h</header>
+   <container>1</container>
+  </customwidget>
  </customwidgets>
  <resources/>
  <connections/>

kcmkwin/kwindecoration/kwindecoration.cpp

@@ -380,6 +380,15 @@ void KWinDecorationModule::slotConfigureDecoration()
         form->borderSizesCombo->setCurrentIndex(index.data(DecorationModel::BorderSizeRole).toInt());
         form->buttonSizesCombo->setCurrentIndex(index.data(DecorationModel::ButtonSizeRole).toInt());
         form->closeWindowsDoubleClick->setChecked(index.data(DecorationModel::CloseOnDblClickRole).toBool());
+        form->doubleClickMessage->setCloseButtonVisible(true);
+        form->doubleClickMessage->setText(i18n("Close by double clicking:\n To open the menu, keep the button pressed until it appears."));
+        form->doubleClickMessage->setVisible(false);
+        connect(form->closeWindowsDoubleClick, &QCheckBox::toggled, [form](bool toggled) {
+            if (!toggled) {
+                return;
+            }
+            form->doubleClickMessage->animatedShow();
+        });
         // in case of QmlDecoration look for a config.ui in the package structure
         KConfigDialogManager *configManager = nullptr;
         if (index.data(DecorationModel::TypeRole).toInt() == DecorationModelData::QmlDecoration) {