Add mouse wheel handlign 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.

Closes https://github.com/wxWidgets/wxWidgets/pull/88

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);
 

src/generic/calctrlg.cpp

@@ -59,6 +59,7 @@ wxBEGIN_EVENT_TABLE(wxGenericCalendarCtrl, wxControl)
 
     EVT_LEFT_DOWN(wxGenericCalendarCtrl::OnClick)
     EVT_LEFT_DCLICK(wxGenericCalendarCtrl::OnDClick)
+    EVT_MOUSEWHEEL(wxGenericCalendarCtrl::OnWheel)
 
     EVT_SYS_COLOUR_CHANGED(wxGenericCalendarCtrl::OnSysColourChanged)
 wxEND_EVENT_TABLE()
@@ -1512,6 +1513,28 @@ wxCalendarHitTestResult wxGenericCalendarCtrl::HitTest(const wxPoint& pos,
     }
 }
 
+void wxGenericCalendarCtrl::OnWheel(wxMouseEvent& event)
+{
+    wxDateSpan span;
+    switch ( event.GetWheelAxis() )
+    {
+        case wxMOUSE_WHEEL_VERTICAL:
+            span = wxDateSpan::Month();
+            break;
+
+        case wxMOUSE_WHEEL_HORIZONTAL:
+            span = wxDateSpan::Year();
+            break;
+    }
+
+    // Currently we only take into account the rotation direction, not its
+    // magnitude.
+    if ( event.GetWheelRotation() < 0 )
+        span = -span;
+
+    SetDateAndNotify(m_date + span);
+}
+
 // ----------------------------------------------------------------------------
 // subcontrols events handling
 // ----------------------------------------------------------------------------