Support classFqn for @StudioElementsGroup

[ilushaaa]

<component>
  <elementGroup1>
    <elementGroup2>
      <element/>
      <element/>
    </elementGroup2>
  </elementGroup1>
</component>

For this descriptor structure <elementGroup2> cannot be appended to <elementGroup1> because <elementGroup1> doesn't have a classFqn and there is nothing to specify in the <elementGroup2> target . As a result it is impossible to add <element> at once in the Inspector without manually adding <elementGroup1>.