Add enabled/disable support for CheckBoxes, RadioButtons and Switches plasma components

- The current disabled CheckBoxes, RadioButtons and Switches,
   have less opacity when disabled instead of custom graphics.
   Needs to be defined if this is the expected behaviour.

Signed-off-by: Daker Fernandes Pinheiro <dakerfp@gmail.com>

declarativeimports/plasmacomponents/DualStateButton.qml

@@ -36,15 +36,20 @@ Item {
 
     width: surface.width + label.paintedWidth
     height: surface.height
+    opacity: dualButton.enabled ? 1.0 : 0.5 // XXX: temporary solution
 
     function entered() {
-        shadow.opacity = 0;
-        hover.opacity = 1;
+        if (dualButton.enabled) {
+            shadow.opacity = 0;
+            hover.opacity = 1;
+        }
     }
 
     function released() {
-        dualButton.checked = !dualButton.checked;
-        dualButton.clicked();
+        if (dualButton.enabled) {
+            dualButton.checked = !dualButton.checked;
+            dualButton.clicked();
+        }
     }
 
     Keys.onSpacePressed: entered();

declarativeimports/test/gallery/CheckableButtons.qml

@@ -46,7 +46,8 @@ Column {
 
     PlasmaComponents.CheckBox {
         height: 30
-        text: "Check Box 2"
+        text: "Disabled"
+        enabled: false
     }
 
     PlasmaComponents.CheckBox {