[wayland] AbstractBackend indicates whether outputs are enabled

By default Outputs are considered to be enabled. This allows a
concrete backend implementation to mark outputs as disabled (e.g.
if dpms has put outputs into powersaving mode). This allows the
Compositor to disable itself if outputs are not visible anyway.

abstract_backend.h

@@ -86,6 +86,9 @@ public:
     bool supportsPointerWarping() const {
         return m_pointerWarping;
     }
+    bool areOutputsEnabled() const {
+        return m_outputsEnabled;
+    }
 
 public Q_SLOTS:
     void pointerMotion(const QPointF &position, quint32 time);
@@ -128,6 +131,9 @@ protected:
     void setSupportsPointerWarping(bool set) {
         m_pointerWarping = set;
     }
+    void setOutputsEnabled(bool enabled) {
+        m_outputsEnabled = enabled;
+    }
 
 private:
     void triggerCursorRepaint();
@@ -144,6 +150,7 @@ private:
     QSize m_initialWindowSize;
     QByteArray m_deviceIdentifier;
     bool m_pointerWarping = false;
+    bool m_outputsEnabled = true;
 };
 
 }