Follow-up to 3622f06bed

Work around 3D scene focus after de-activation of the main
window without having to resort to CallAfter(), which breaks
on Linux with some window managers that follow mouser cursor.
Fixes #5620 #6870 #6992

3622f06bed was not a correct solution,
it broke focus for non-modal windows.
Fixes #7419

The actual issue seems to be caused by wxProgressDialog not playing
well with modal dialogs closed just before wxProgressDialog opens.
If wxProgressDialog parent was not a main frame, keyboard focus
was not restored correctly after the wxProgressDialog closed.

src/slic3r/GUI/GCodeViewer.cpp

@@ -1447,7 +1447,7 @@ void GCodeViewer::load_toolpaths(const GCodeProcessorResult& gcode_result)
     static const unsigned int progress_threshold = 1000;
     wxProgressDialog* progress_dialog = wxGetApp().is_gcode_viewer() ?
         new wxProgressDialog(_L("Generating toolpaths"), "...",
-            100, wxGetApp().plater(), wxPD_AUTO_HIDE | wxPD_APP_MODAL) : nullptr;
+            100, wxGetApp().mainframe, wxPD_AUTO_HIDE | wxPD_APP_MODAL) : nullptr;
 
     wxBusyCursor busy;