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));
 

src/slic3r/GUI/Field.cpp

@@ -291,6 +291,16 @@ void Field::get_value_by_opt_type(wxString& str, const bool check_value/* = true
 	case coString:
 	case coStrings:
     case coFloatOrPercent: {
+        if (m_opt.type == coFloatOrPercent && m_opt.opt_key == "first_layer_height" && !str.IsEmpty() && str.Last() == '%') {
+            // Workaroud to avoid of using of the % for first layer height
+            // see https://github.com/prusa3d/PrusaSlicer/issues/7418
+            wxString label = m_opt.full_label.empty() ? _(m_opt.label) : _(m_opt.full_label);
+            show_error(m_parent, from_u8((boost::format(_utf8(L("%s doesn't support percentage"))) % label).str()));
+            const wxString stVal = double_to_string(0.01, 2);
+            set_value(stVal, true);
+            m_value = into_u8(stVal);;
+            break;
+        }
         if (m_opt.type == coFloatOrPercent && !str.IsEmpty() &&  str.Last() != '%')
         {
             double val = 0.;

src/slic3r/GUI/OG_CustomCtrl.cpp

@@ -167,13 +167,14 @@ wxPoint OG_CustomCtrl::get_pos(const Line& line, Field* field_in/* = nullptr*/)
                 break;
             }
 
+            bool is_multioption_line = option_set.size() > 1;
             for (auto opt : option_set) {
                 Field* field = opt_group->get_field(opt.opt_id);
                 correct_line_height(ctrl_line.height, field->getWindow());
 
                 ConfigOptionDef option = opt.opt;
                 // add label if any
-                if (!option.label.empty()) {
+                if (is_multioption_line && !option.label.empty()) {
                     //!            To correct translation by context have to use wxGETTEXT_IN_CONTEXT macro from wxWidget 3.1.1
                     label = (option.label == L_CONTEXT("Top", "Layers") || option.label == L_CONTEXT("Bottom", "Layers")) ?
                         _CTX(option.label, "Layers") : _(option.label);
@@ -595,11 +596,12 @@ void OG_CustomCtrl::CtrlLine::render(wxDC& dc, wxCoord v_pos)
     }
 
     size_t bmp_rect_id = 0;
+    bool is_multioption_line = option_set.size() > 1;
     for (const Option& opt : option_set) {
         field = ctrl->opt_group->get_field(opt.opt_id);
         ConfigOptionDef option = opt.opt;
         // add label if any
-        if (!option.label.empty()) {
+        if (is_multioption_line && !option.label.empty()) {
             //!            To correct translation by context have to use wxGETTEXT_IN_CONTEXT macro from wxWidget 3.1.1
             label = (option.label == L_CONTEXT("Top", "Layers") || option.label == L_CONTEXT("Bottom", "Layers")) ?
                     _CTX(option.label, "Layers") : _(option.label);

src/slic3r/GUI/OptionsGroup.cpp

@@ -507,15 +507,13 @@ void OptionsGroup::clear(bool destroy_custom_ctrl)
 	m_fields.clear();
 }
 
-Line OptionsGroup::create_single_option_line(const Option& option, const std::string& path/* = std::string()*/) const {
+Line OptionsGroup::create_single_option_line(const Option& option, const std::string& path/* = std::string()*/) const
-// 	Line retval{ _(option.opt.label), _(option.opt.tooltip) };
+{
     wxString tooltip = _(option.opt.tooltip);
     edit_tooltip(tooltip);
 	Line retval{ _(option.opt.label), tooltip };
 	retval.label_path = path;
-    Option tmp(option);
+    retval.append_option(option);
-    tmp.opt.label = std::string("");
-    retval.append_option(tmp);
     return retval;
 }