Added a check that first layer height is not expressed as a percentage (related to https://github.com/prusa3d/PrusaSlicer/issues/7418)

first_layer_height cannot be changed to ConfigOptionFloat, that would break loading of old 3MFs.
The relative values from 3MFs should already be converted to absolute in `Preset::normalize`, what is missing is the UI check.

+ Code refactoring for OptionsGroup::create_single_option_line():
Don't clear label value in an Option. This value is used in Field::get_value_by_opt_type() to show error "%s doesn't support percentage".
=> At functions OG_CustomCtrl::CtrlLine::render() and OG_CustomCtrl::CtrlLine::get_pos() added check if current line has more than one option.
=> Draw option's label only when line has several options.

src/libslic3r/PrintConfig.cpp

@@ -1210,6 +1210,7 @@ void PrintConfigDef::init_fff_params()
     def->tooltip = L("When printing with very low layer heights, you might still want to print a thicker "
                    "bottom layer to improve adhesion and tolerance for non perfect build plates.");
     def->sidetext = L("mm");
+    def->min = 0;
     def->ratio_over = "layer_height";
     def->set_default_value(new ConfigOptionFloatOrPercent(0.35, false));