Duplicate results for menu entries if Layout is provided.

If a layout is included for a menu, all entries that would be included in <Merge type="menus"/> or <Merge type="files"/> is included twice.

The culprit is this method:

@property
def order(self):
    return self._order if self._order else self._default_order

Layout._order is never actually set. Instead we append to the result of Layout.order. With the result that we add any layout order rules after the default order rules, instead of replacing them. I'll make a PR to fix this in the next few days.