Ignore time component of SYSTEMTIME in wxCalendarCtrl.

Ignore time component of SYSTEMTIME in wxCalendarCtrl.

Native month calendar functions doesn't always return correct values in the
time part of SYSTEMTIME so ignore it and use just the date component.

To simplify doing it, add helper (MSW-specific) SetFromMSWSysDate() and
GetAsMSWSysDate() functions which convert between wxDateTime and SYSTEMTIME
but take only date component into account.

This commit partially replaces changes of r63560 and closes #11276.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@64208 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775