Replace backgroundColor with backgroundVisible

This exposes backgroundVisible to TableView and
replaces backgroundColor in TextArea.

We think the backgroundVisible is more flexible and allows us
to find a more appropriate solution to support palette in the
future.

Change-Id: Icfcd512c29cf030059c4ce2d32feb402d1a33662
Reviewed-by: Caroline Chao <caroline.chao@digia.com>
Reviewed-by: J-P Nurmi <jpnurmi@digia.com>

src/controls/TableView.qml

@@ -123,6 +123,16 @@ ScrollView {
         The default value is \c true. */
     property bool headerVisible: true
 
+    /*! \qmlproperty bool TableView::backgroundVisible
+
+        This property determines if the background should be filled or not.
+
+        The default value is \c true.
+
+        \note The rowDelegate is not affected by this property
+    */
+    property alias backgroundVisible: colorRect.visible
+
     /*! This property defines a delegate to draw a specific cell.
 
     In the item delegate you have access to the following special properties:

src/controls/TextArea.qml

@@ -599,14 +599,13 @@ ScrollView {
         edit.undo();
     }
 
-    /*!
-        \qmlproperty color TextArea::backgroundColor
+    /*! \qmlproperty bool TextArea::backgroundVisible
 
-        This property sets the background color of the viewport.
+        This property determines if the background should be filled or not.
 
-        The default value is the base color of the SystemPalette.
+        The default value is \c true.
     */
-    property alias backgroundColor: colorRect.color
+    property alias backgroundVisible: colorRect.visible
 
     /*! \internal */
     default property alias data: area.data