added support for wxDP_ALLOWNONE in wxMSW; documented it; added test for it in the sample

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@31987 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775

docs/latex/wx/datectrl.tex

@@ -44,6 +44,9 @@ calendar drop down part from which the user can select a date.}
 \twocolitem{\windowstyle{wxDP\_DEFAULT}}{Creates a control with default style
 which is the best supported for the current platform (currently wxDP\_SPIN
 under Windows and wxDP\_DROPDOWN elsewhere).}
+\twocolitem{\windowstyle{wxDP\_ALLOWNONE}}{With this style, the control allows
+the user to not enter any valid date at all. Without it -- which is by default
+-- the control always has some valid date.}
 \twocolitem{\windowstyle{wxDP\_SHOWCENTURY}}{Forces display of the century in
 the default date format. Without this flas the century could be displayed or
 not depending on the default date representation in the system.}