Menu default selection

Need a mechanism to specify a default MenuItem.

The return value of Menu::operator() is currently a bool, where true indicates a normal item was selected, and false indicates BACK was selected.

Ideas:

1. Change the return type of Menu::operator() to int. 0 would represent BACK, and other numbers would represent that numbered element in the array + 1. That would allow us to keep the "truthy" and "falsy" behavior that's already being used, while adding the extra capability. Or
2. Add a mechanism to MenuItem to specify this item as the default. Though there would be no way to enforce only one MenuItem is listed as default.