Add dropdown for Label-type. Remove Treeview. Reorganize dialog in order to save as much space as possible for the GLWidget. Many problems in this ui, I hope somebody can help me. How do I tell the GLWidget to fill all the empty space?

Rationale for removing the Treeview is that
1) libavogadro has its own treeview, so we should use it (and perhaps improve it) if we want a treeview
2) a treeview is mostly useful when selection works, and we had agreed with cryos that selection is not for 4.0 but only for 4.1. The selectrotate tool is not ready. And selection is mostly useful in an editor, not in a mere viewer.