Add support for date format to wxGridCellDateEditor too

This is needed to allow editing the cells using wxGridCellDateRenderer
with a custom format, otherwise the editor might parse the contents of
the cell differently from what is actually shown.

In particular, this ensures that using "date:%d.%m.%y" (or any other
custom format) as "cell type" works correctly, which wasn't the case
before.

interface/wx/grid.h

@@ -1092,8 +1092,14 @@ class wxGridCellDateEditor : public wxGridCellEditor
 public:
     /**
         Date editor constructor.
+
+        @param format Optional format for the date displayed in the associated
+            cell. By default, the locale-specific date format ("%x") is assumed.
+            You would typically want to specify the same format as the one
+            used with the cell renderer, if a non-default one is used.
+            Note that this parameter is only available since wxWidgets 3.1.5.
     */
-    wxGridCellDateEditor();
+    explicit wxGridCellDateEditor(const wxString& format = wxString());
 };