Merge upstream main: multi-variant printers, X-Ray view, preview and startup work

The two conflicts are both places where upstream landed on top of IDEX/IQEX
code. In GCode.cpp the relocated file header block meets the IMEX placeholder
block, and the placeholders are set first: file_start_gcode is processed through
the placeholder parser now, which throws on a name it does not know, so a script
naming {imex_mode} would abort the export if the header ran first. In
test_multifilament.cpp both sides appended a case at the end of the file.

src/slic3r/GUI/GCodeViewer.cpp

@@ -1452,6 +1452,7 @@ void GCodeViewer::load_as_gcode(const GCodeProcessorResult& gcode_result, const
             libvgcode::EGCodeExtrusionRole::SupportTransition, libvgcode::EGCodeExtrusionRole::Mixed
             });
     m_paths_bounding_box = BoundingBoxf3(libvgcode::convert(bbox[0]).cast<double>(), libvgcode::convert(bbox[1]).cast<double>());
+    m_max_bounding_box = m_paths_bounding_box;
 
     if (wxGetApp().is_editor())
         m_contained_in_bed = wxGetApp().plater()->build_volume().all_paths_inside(gcode_result, m_paths_bounding_box);