Show wait cursor when exporting an STL.

Fix of SLA gizmo picking by rectangle: point is occluded only if an object
is half its radius away from the point center.
Don't show the temp G-code file name in the status bar when exporting.

src/slic3r/GUI/Plater.cpp

@@ -2972,6 +2972,8 @@ void Plater::export_stl(bool selection_only)
     const wxString path = dialog->GetPath();
     const std::string path_u8 = into_u8(path);
 
+    wxBusyCursor wait;
+
     TriangleMesh mesh;
     if (selection_only) {
         const auto &selection = p->get_selection();