[decorations] Allow decorations to use a QWindow instead of QWidget

A setMainWindow() method is added which behaves similar to
setMainWidget(). In addition a few convenient methods are added which
can be used by KWin core to show/hide the decoration without caring
whether the decoration uses a QWindow or QWidget.

libkdecorations/kdecoration.cpp

@@ -109,11 +109,46 @@ void KDecoration::setMainWidget(QWidget* w)
     widget()->resize(geometry().size());
 }
 
+void KDecoration::setMainWindow(QWindow *window)
+{
+    assert(d->window.isNull());
+    d->window.reset(window);
+    d->window->resize(geometry().size());
+}
+
 Qt::WindowFlags KDecoration::initialWFlags() const
 {
     return d->bridge->initialWFlags();
 }
 
+void KDecoration::show()
+{
+    if (!d->w.isNull()) {
+        d->w->show();
+    } else if (!d->window.isNull()) {
+        d->window->show();
+    }
+}
+
+void KDecoration::hide()
+{
+    if (!d->w.isNull()) {
+        d->w->hide();
+    } else if (!d->window.isNull()) {
+        d->window->hide();
+    }
+}
+
+QRect KDecoration::rect() const
+{
+    if (!d->w.isNull()) {
+        return d->w->rect();
+    } else if (!d->window.isNull()) {
+        return QRect(QPoint(0, 0), d->window->size());
+    }
+    return QRect();
+}
+
 bool KDecoration::isActive() const
 {
     return d->bridge->isActive();

libkdecorations/kdecoration.h

@@ -982,6 +982,17 @@ public:
      *              like WX11BypassWM or WStyle_NoBorder are forbidden.
      */
     void createMainWidget(Qt::WindowFlags flags = 0);
+    /**
+     * This should be the first function called in init() to specify
+     * the main window of the decoration. The window should be created
+     * with window flags specified by initialWFlags().
+     *
+     * This class takes over ownership of the QWindow.
+     *
+     * A window decoration can either be QWidget based or QWindow based. If it
+     * calls both setMainWidget() and setMainWindow() the behavior is undefined.
+     */
+    void setMainWindow(QWindow *window);
     /**
      * The flags that should be used when creating the main widget.
      * It is possible to add more flags when creating the main widget, but only flags
@@ -1022,6 +1033,18 @@ public:
      * a button press was for window tab dragging or for displaying the client menu.
      */
     WindowOperation buttonToWindowOperation(Qt::MouseButtons button);
+    /**
+     * Convenient method to show the decoration's widget or window.
+     **/
+    void show();
+    /**
+     * Convenient method to hide the decoration's widget or window.
+     **/
+    void hide();
+    /**
+     * Convenient method to get the geometry of the decoration widget or window.
+     **/
+    QRect rect() const;
 
 
     // Window tabbing