Export WindowId on Plasma Dialog

Exporting the window Id allows to use it in declarative KWin scripts
to embedd window thumbnails on the dialog.

REVIEW: 104393

declarativeimports/core/dialog.cpp

@@ -350,6 +350,11 @@ int DialogProxy::windowFlags() const
     return (int)m_flags;
 }
 
+qulonglong DialogProxy::windowId() const
+{
+    return m_dialog->winId();
+}
+
 void DialogProxy::setWindowFlags(const int flags)
 {
     /*X misbehaviour: the only way to make a window with the Popup flag working, is to create it with that flag from the beginning*/

declarativeimports/core/dialog.h

@@ -140,6 +140,11 @@ class DialogProxy : public QObject
      */
     Q_PROPERTY(int location READ location WRITE setLocation NOTIFY locationChanged)
 
+    /**
+     * Window ID of the dialog window.
+     **/
+    Q_PROPERTY(qulonglong windowId READ windowId CONSTANT)
+
 public:
     enum WidgetAttribute {
         WA_X11NetWmWindowTypeDock = Qt::WA_X11NetWmWindowTypeDock
@@ -180,6 +185,8 @@ public:
 
     QObject *margins() const;
 
+    qulonglong windowId() const;
+
     /**
      * @returns The suggested screen position for the popup
      * @arg item the item the popup has to be positioned relatively to. if null, the popup will be positioned in the center of the window