Rework lookup of sub-elements with regard to special accepted elements

[ogoffart]

Consider

// Some code in the wild actually have made their own menubar before
component MenuBar { /* ... */ }

component Row { /* ... */ }

export component Main inherits Window {
  // This should find the declared component and not the builtin component
  MenuBar { }

  GridLayout {
     Row {}
  }
}

Previously, MenuBar and Row were first resolved against additional_accepted_child_types and so it would always find the builtin one.
But this patch changes that to find the builtin one only if it hasn't been replaced by another component with the name name. (So we'd use the cusom MenuBar and the custom Row)

Another change is that we also lookup the
additional_accepted_child_types if they are in a @children.

component MyGrid {
   GridLayout { @children }
}
export component Main {
   MyGrid {
      // This used to be an error but is now accepted
      Row { }
   }
}