Slightly improve the date/time dialogs in the calendar sample.

Use CreateStdDialogButtonSizer() instead of constructing
wxStdDialogButtonSizer directly, not only this is shorter but it also makes
the "OK" button default correctly.

Also initialize the labels instead of leaving them initially empty.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@72898 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775