[Sketch] fix SnellsLaw dialog

- add missing class to UI file (found and automatically added by Qt Designer)
- add more sensible stepsize - you are dealing in practice with rations in the range 1.2 - 1.6 thus a stepsize of 1.0 is not helpful

src/Mod/Sketcher/Gui/CommandConstraints.cpp

@@ -6976,6 +6976,7 @@ void CmdSketcherConstrainSnellsLaw::activated(int iMsg)
         ui_Datum.labelEdit->setParamGrpPath(QByteArray("User parameter:BaseApp/History/SketcherRefrIndexRatio"));
         ui_Datum.labelEdit->setToLastUsedValue();
         ui_Datum.labelEdit->selectNumber();
+        ui_Datum.labelEdit->setSingleStep(0.05);
         // Unable to bind, because the constraint does not yet exist
 
         if (dlg.exec() != QDialog::Accepted) return;

src/Mod/Sketcher/Gui/EditDatumDialog.cpp

@@ -119,6 +119,7 @@ void EditDatumDialog::exec(bool atCursor)
             dlg.setWindowTitle(tr("Refractive index ratio", "Constraint_SnellsLaw"));
             ui_ins_datum->label->setText(tr("Ratio n2/n1:", "Constraint_SnellsLaw"));
             ui_ins_datum->labelEdit->setParamGrpPath(QByteArray("User parameter:BaseApp/History/SketcherRefrIndexRatio"));
+            ui_ins_datum->labelEdit->setSingleStep(0.05);
         }
         else {
             dlg.setWindowTitle(tr("Insert length"));

src/Mod/Sketcher/Gui/InsertDatum.ui

@@ -33,7 +33,7 @@
       </widget>
      </item>
      <item row="0" column="1">
-      <widget class="Gui::PrefQuantitySpinBox" name="labelEdit" native="true">
+      <widget class="Gui::PrefQuantitySpinBox" name="labelEdit">
        <property name="sizePolicy">
         <sizepolicy hsizetype="MinimumExpanding" vsizetype="Preferred">
          <horstretch>0</horstretch>
@@ -94,8 +94,13 @@
  </widget>
  <customwidgets>
   <customwidget>
-   <class>Gui::PrefQuantitySpinBox</class>
+   <class>Gui::QuantitySpinBox</class>
    <extends>QWidget</extends>
+   <header>Gui/QuantitySpinBox.h</header>
+  </customwidget>
+  <customwidget>
+   <class>Gui::PrefQuantitySpinBox</class>
+   <extends>Gui::QuantitySpinBox</extends>
    <header>Gui/PrefWidgets.h</header>
    <slots>
     <signal>showFormulaDialog(bool)</signal>