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/window.h

@@ -206,8 +206,8 @@ public:
     void SubclassWin(WXHWND hWnd);
     void UnsubclassWin();
 
-    WXFARPROC MSWGetOldWndProc() const { return m_oldWndProc; }
-    void MSWSetOldWndProc(WXFARPROC proc) { m_oldWndProc = proc; }
+    WXWNDPROC MSWGetOldWndProc() const { return m_oldWndProc; }
+    void MSWSetOldWndProc(WXWNDPROC proc) { m_oldWndProc = proc; }
 
     // return true if the window is of a standard (i.e. not wxWidgets') class
     //
@@ -597,7 +597,7 @@ protected:
     WXHWND                m_hWnd;
 
     // the old window proc (we subclass all windows)
-    WXFARPROC             m_oldWndProc;
+    WXWNDPROC             m_oldWndProc;
 
     // additional (MSW specific) flags
     bool                  m_mouseInWindow:1;