Add WXWNDPROC typedef and use it in wxMSW instead of WXFARPROC

WNDPROC and FARPROC are not the same thing in MSW and it's wrong to use
WXFARPROC as the type of different window procedures we use.

Introduce WXWNDPROC which is more clear and correct and use it instead.

Also get rid of a few casts which are not necessary any longer.

src/msw/spinctrl.cpp

@@ -329,8 +329,7 @@ bool wxSpinCtrl::Create(wxWindow *parent,
     // subclass the text ctrl to be able to intercept some events
     gs_spinForTextCtrl[GetBuddyHwnd()] = this;
 
-    m_wndProcBuddy = (WXFARPROC)wxSetWindowProc(GetBuddyHwnd(),
-                                                wxBuddyTextWndProc);
+    m_wndProcBuddy = wxSetWindowProc(GetBuddyHwnd(), wxBuddyTextWndProc);
 
     // associate the text window with the spin button
     (void)::SendMessage(GetHwnd(), UDM_SETBUDDY, (WPARAM)m_hwndBuddy, 0);