Implemented handling of complex ConfigOptionFloatOrPercent chains

by the PlaceholderParser.

Namely, all the options with the "ratio_over" reference are now handled
correctly by the PlaceholderParser with the exception
of the "first_layer_extrusion_width", which overrides speed of extrusions
by their respective extrusion type.

Also the various extrusion widths (extrusion_width, first_layer_extrusion_width,
external_perimeter_extrusion_width etc.) produce the same numbers
as if ran through the back-end, with the assumption of not overriding
layer height by the variable layer height editing tool or layer height
modifiers.

src/libslic3r/PrintConfig.cpp

@@ -718,8 +718,9 @@ void PrintConfigDef::init_fff_params()
     def->gui_type = "f_enum_open";
     def->gui_flags = "show_value";
     def->enum_values.push_back("PLA");
-    def->enum_values.push_back("ABS");
     def->enum_values.push_back("PET");
+    def->enum_values.push_back("ABS");
+    def->enum_values.push_back("ASA");
     def->enum_values.push_back("FLEX");
     def->enum_values.push_back("HIPS");
     def->enum_values.push_back("EDGE");