[TD] sanitize Scale preferences

- take care that custom scale is only in effect if the scale type is 'Custom'
- use UnitSpinBox for Template Mark
- use uniform width for all scale edits
- also use UnitSpinBox for Balloon Kink

src/Mod/TechDraw/Gui/DlgPrefsTechDraw3.ui

@@ -716,7 +716,7 @@
          </widget>
         </item>
         <item row="7" column="2">
-         <widget class="Gui::PrefDoubleSpinBox" name="pdsbBalloonKink">
+         <widget class="Gui::PrefUnitSpinBox" name="pdsbBalloonKink">
           <property name="sizePolicy">
            <sizepolicy hsizetype="Preferred" vsizetype="Preferred">
             <horstretch>0</horstretch>