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>