Add mouse wheel handling to wxGenericCalendarCtrl

Mimics the scrolling behaviour of native MSW and GTK calendars by allowing to
increment/decrement the month by scrolling anywhere on the generic calendar.

Additionally, use horizontal scrolling to increment/decrement the year.

Backport of 50daf1feab + 4177593aef
into WX_3_0_BRANCH.

See also 50daf1feab (commitcomment-13087042)

include/wx/generic/calctrlg.h

@@ -179,6 +179,7 @@ private:
     void OnPaint(wxPaintEvent& event);
     void OnClick(wxMouseEvent& event);
     void OnDClick(wxMouseEvent& event);
+    void OnWheel(wxMouseEvent& event);
     void OnChar(wxKeyEvent& event);
     void OnMonthChange(wxCommandEvent& event);