Make buttons respond to accessible press.

QML accessibility looks for and calls accessiblePressAction
when a Press action is received.

Change-Id: I53779ba0f6833fde3d593ee5ac6a4106fd9d3521
Reviewed-by: J-P Nurmi <jpnurmi@digia.com>

src/private/BasicButton.qml

@@ -115,6 +115,11 @@ Control {
     Accessible.role: Accessible.Button
     Accessible.description: tooltip
 
+    /*! \internal */
+    function accessiblePressAction() {
+        __action.trigger()
+    }
+
     Action { id: ownAction }
 
     Connections {