Add wxDateTimePickerCtrl::SetNullText()

This allows to customize the string shown when there is no valid date
under MSW (only, for now) and can be notably used to suppress the
unused date completely, which can be useful to lighten up the display
when there are many controls.

Add UI elements to the widgets sample allowing to test the new function.

interface/wx/datectrl.h

@@ -168,6 +168,22 @@ public:
     */
     virtual wxDateTime GetValue() const;
 
+    /**
+        Set the text to show when there is no valid value.
+
+        For the controls with @c wxDP_ALLOWNONE style, set the string displayed
+        when the control doesn't have any valid value. Currently this is only
+        actually used under MSW, where it can be used to override the previous
+        value which is still displayed by the control in this case, and ignored
+        elsewhere.
+
+        Notably, @a text can be empty to completely hide the date if no valid
+        date is specified.
+
+        @since 3.1.5
+     */
+    void SetNullText(const wxString& text);
+
     /**
         Sets the valid range for the date selection. If @a dt1 is valid, it
         becomes the earliest date (inclusive) accepted by the control. If