PATCH: Using wxDatePickerCtrl instead of wxCalendarBox

For some reason, the wxCalendarBox control was not working on linux (i.e. Ubuntu).

I found that - wxWidgets has already provided a class for that. (wxDatePickerCtrl).

As per our conversation, I have modified the code to use the wxDatePickerCtrl instead of wxCalendarBox.

Please find the patch for the master branch.

I have made minimal changes to work this control as per our expectation.

I think - this is a bug, so we should apply this to REL-1_12 and REL-1_10 branches too...

If you want, I can generate the patches for them.

Please review this patch and give your comments.