Add enabled/disable support for Button plasma component

- The current disabled buttons 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/Button.qml

@@ -37,10 +37,14 @@ Item {
     property QtObject theme: PlasmaCore.Theme { }
 
     function pressButton() {
+        if (button.enabled)
             surface.prefix = "pressed";
     }
 
     function releaseButton() {
+        if (!button.enabled)
+            return;
+
         if (button.checkable)
             button.checked = !button.checked;
 
@@ -56,6 +60,7 @@ Item {
 
     width: 50
     height: 20
+    opacity: enabled ? 1.0 : 0.5 // XXX: temporary solution
 
     Keys.onSpacePressed: pressButton();
     Keys.onReturnPressed: pressButton();

declarativeimports/test/gallery/Buttons.qml

@@ -89,4 +89,11 @@ Column {
 
         Keys.onTabPressed: bt1.forceActiveFocus();
     }
+
+    PlasmaComponents.Button {
+        width: 140
+        height: 30
+        text: "Disabled Button"
+        enabled: false
+    }
 }