Add a simple wxSetWindowFont() helper

This function is a just a very thin wrapper for WM_SETFONT, but it's
still better to have it rather than write casts to WPARAM and
MAKELPARAM() in several different places.

Note that this removes the assert for font validity from
wxWindow::SetFont() which really doesn't make much sense (and if we
wanted to have it, it would be better to have it for all ports in
wxWindowBase instead) and was never triggered since more than 20 years
of its existence.

src/msw/listctrl.cpp

@@ -620,8 +620,7 @@ bool wxListCtrl::SetHeaderAttr(const wxItemAttr& attr)
 
             // We need to tell the header about its new font to let it compute
             // its new height.
-            ::SendMessage(hwndHdr, WM_SETFONT,
-                          (WPARAM)GetHfontOf(font), MAKELPARAM(TRUE, 0));
+            wxSetWindowFont(hwndHdr, font);
         }
 
         // Refreshing the listview makes it notice the change in height of its