Use enum for wxMouseEvent::m_wheelAxis instead of int.

This variable can take only 2 values, use symbolic names for them instead of
difficult to understand 0 and 1.

See ##14105.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@70961 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775

src/common/event.cpp

@@ -554,10 +554,10 @@ wxMouseEvent::wxMouseEvent(wxEventType commandType)
 
     m_clickCount = -1;
 
+    m_wheelAxis = wxMOUSE_WHEEL_VERTICAL;
     m_wheelRotation = 0;
     m_wheelDelta = 0;
     m_linesPerAction = 0;
-    m_wheelAxis = 0;
 }
 
 void wxMouseEvent::Assign(const wxMouseEvent& event)