Improved the FDM supports gizmo dialog

Removed unused code
Fixed a clipping-plane related crash
Fixed a crash in hollowing gizmo when no hollowed mesh was provided
Forbid opening the gizmo when a part of an object is selected

src/slic3r/GUI/Gizmos/GLGizmoHollow.cpp

@@ -59,7 +59,7 @@ void GLGizmoHollow::set_sla_support_data(ModelObject*, const Selection&)
     const ModelObject* mo = m_c->selection_info()->model_object();
     if (mo) {
         reload_cache();
-        if (m_c->hollowed_mesh()->get_hollowed_mesh())
+        if (m_c->hollowed_mesh() && m_c->hollowed_mesh()->get_hollowed_mesh())
             m_holes_in_drilled_mesh = mo->sla_drain_holes;
     }
 }