Add a 'setAddBackground' method to SlidingMenu to let us avoid the autom...

...atic addition of the window's background behind the content view. This lets us avoid overdraw, which should make redraw faster.

The changes in https://github.com/jfeinstein10/SlidingMenu/pull/294 didn't really work for me. I think it's important that the current behaviour, i.e. you don't need to explicitly set a background on the content view or the menu view, is the default as it makes the library easier to use. But if you want to remove unnecessary overdraw I don't see any alternative other than setting the window background to null, and if you do that then you obviously need to have the content/menu views have their own backgrounds.

I've tweaked the Changing Fragments example to show how the new method would work, and this eliminates the unnecessary overdraw for this case.