disable previous and next button

if there is no corresponding media; set play/pause symbol via model
property instead of via qml.

LenaPi/RoundButton.qml

@@ -21,6 +21,15 @@ Rectangle {
         height: width
         source: "qrc:/icon_back"
     }
+    Rectangle{
+        id: overlay
+        z: 1
+        visible: !container.enabled
+        anchors.centerIn: parent
+        width: 30
+        height: width
+        color: "#99ffffff"
+    }
 
     MouseArea{
         id: controlObject