Thomas Arnhold asked me to take a look at the method SetBackgroundBitmap.
In my XFCE, removing this method didn't chnage anything. So this needs more
tests in Windows and other WM's.

Change-Id: I3e10bea4eac114326ff981fb21ba0d292818b1da
Signed-off-by: Marcos Paulo de Souza <marcos.souza.org@gmail.com>