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.

include/wx/msw/spinctrl.h

@@ -100,7 +100,7 @@ public:
     // for internal use only
 
     // get the subclassed window proc of the buddy text
-    WXFARPROC GetBuddyWndProc() const { return m_wndProcBuddy; }
+    WXWNDPROC GetBuddyWndProc() const { return m_wndProcBuddy; }
 
     // return the spinctrl object whose buddy is the given window or NULL
     static wxSpinCtrl *GetSpinForTextCtrl(WXHWND hwndBuddy);
@@ -145,7 +145,7 @@ protected:
 
     // the data for the "buddy" text ctrl
     WXHWND     m_hwndBuddy;
-    WXFARPROC  m_wndProcBuddy;
+    WXWNDPROC  m_wndProcBuddy;
 
     // Block text update event after SetValue()
     bool m_blockEvent;