Get dates directly from table in wxGridCellDateEditor if possible

Unlike wxGridCellDateRenderer, which already did it, the editor class
always got the cell value from the table as a string, even if the table
supported returning the dates directly.

Fix this by using the same code in the editor as in the renderer, which
required a further refactoring in order to make it reusable: the helper
TryParseDate() was replaced with TryGetValueAsDate() and DateParseParams
was added to allow overriding the arguments passed to it in the
overridden wxGridCellDateTimeRenderer::GetDateParseParams().

include/wx/generic/private/grid.h

@@ -1089,11 +1089,51 @@ namespace wxGridPrivate
 
 #if wxUSE_DATETIME
 
-// Helper function trying to parse the given string using the specified date
-// format and then using ParseDate() as a fallback if it failed. If this still
-// fails, returns false.
+// This is used as TryGetValueAsDate() parameter.
+class DateParseParams
+{
+public:
+    // Unfortunately we have to provide the default ctor (and also make the
+    // members non-const) because we use these objects as out-parameters as
+    // they are not fully declared in the public headers. The factory functions
+    // below must be used to create a really usable object.
+    DateParseParams() : fallbackParseDate(false) { }
+
+    // Use these functions to really initialize the object.
+    static DateParseParams WithFallback(const wxString& format)
+    {
+        return DateParseParams(format, true);
+    }
+
+    static DateParseParams WithoutFallback(const wxString& format)
+    {
+        return DateParseParams(format, false);
+    }
+
+    // The usual format, e.g. "%x" or "%Y-%m-%d".
+    wxString format;
+
+    // Whether fall back to ParseDate() is allowed.
+    bool fallbackParseDate;
+
+private:
+    DateParseParams(const wxString& format_, bool fallbackParseDate_)
+        : format(format_),
+          fallbackParseDate(fallbackParseDate_)
+    {
+    }
+};
+
+// Helper function trying to get a date from the given cell: if possible, get
+// the date value from the table directly, otherwise get the string value for
+// this cell and try to parse it using the specified date format and, if this
+// doesn't work and fallbackParseDate is true, try using ParseDate() as a
+// fallback. If this still fails, returns false.
 bool
-TryParseDate(wxDateTime& result, const wxString& text, const wxString& format);
+TryGetValueAsDate(wxDateTime& result,
+                  const DateParseParams& params,
+                  const wxGrid& grid,
+                  int row, int col);
 
 #endif // wxUSE_DATETIME