TextArea.qml: Add backgroundVisible property (http://qt-project.org/doc/qt-5/qml-qtquick-controls-textarea.html#backgroundVisible-prop) so it will be possible to use this componenet to write Notes plasmoid.

Signed-off-by: Davide Bettio <bettio@kde.org>

src/declarativeimports/plasmacomponents/qml/TextArea.qml

@@ -52,6 +52,14 @@ Item {
      */
     property bool errorHighlight
 
+    /**
+     * type:bool
+     * Displays widget background
+     *
+     * The default value is true.
+     */
+    property alias backgroundVisible: base.visible
+
     /**
      * type:int
      * Current cursor position.