Keep displayed month and year in sync in wxGenericCalendarCtrl.

Keep displayed month and year in sync in wxGenericCalendarCtrl.

When the date was constrained to a range in wxGenericCalendarCtrl, the display
of the month in the month combobox could get out of sync with its real value.

Ensure that the correct month is always displayed and also simplify the code
by removing the apparently unnecessarily complex logic in ChangeYear() and
ChangeMonth() functions.

Closes #11060.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@65901 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775