Declare an 'objectHasChildren:' method. Fixes quicksilver/quicksilver#1602

I seem to remember I changed the way loading children worked in v1.1 (Rob probably will too) - if `objectHasChildren:` returns NO (or the method doesn't exist) then `loadObjectForChildren:` is never called.
In this case it broke things, but I still maintain my change was right - the `objectHasChildren:` method *should* be declared - in this case, without declaring it, it was impossible to know which items had children.


Fixes quicksilver/quicksilver#1602