Controls: Rewrite UnitTextBox as extension of TextBox instead of UserControl

Elwig/Windows/AreaComAdminWindow.xaml.cs

@@ -29,7 +29,7 @@ namespace Elwig.Windows {
             ];
             RequiredInputs = [
                 FbNrInput, YearFromInput, KgInput, RdInput,
-                GstNrInput, AreaInput.TextBox, AreaComTypeInput, WineCultivationInput
+                GstNrInput, AreaInput, AreaComTypeInput, WineCultivationInput
             ];
         }