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/GLGizmosCommon.cpp

@@ -306,6 +306,7 @@ void ObjectClipper::on_release()
     m_clipper.reset();
     m_old_mesh = nullptr;
     m_clp.reset();
+    m_clp_ratio = 0.;
 
 }