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/libslic3r/Preset.cpp

@@ -1199,6 +1199,7 @@ static std::vector<std::string> s_Preset_print_options{
     "enable_tower_interface_features",
     "enable_tower_interface_cooldown_during_tower",
     "wipe_tower_no_sparse_layers",
+    "wipe_tower_sparse_layers_combination",
     "compatible_printers",
     "compatible_printers_condition",
     "inherits",