Implemented option to set the priority between the NowPlaying widget and other widgets.

On the main menu, when the user has a widget set to one of the buttons (say, the Latest Movies widget on the Movies button) and the user is playing some media while he's on the main menu the NowPlaying widget should show on the main menu.

It does in fact show, but on the buttons where another widget is set (like the Movies button in the example), that widget is shown instead of the NowPlaying widget.

I've implemented an option which toggles this behavior (see screenshot).
So the user can choose to show the NowPlaying widget instead of a regular widget (either widget1 or widget2, depending on where the NowPlaying widget is shown) or vice versa.

Hopefully the change is good enough to merge in the main branch :)

![screenshot000](https://f.cloud.github.com/assets/3781551/348199/85d74356-9f30-11e2-84b1-ad3d2e9a2b75.png)