Factor out methods for clicking the default button in wxMSW

This will make it possible to reproduce the default "Enter" key functionality
from elsewhere.

Almost no changes yet, the only minor change is that we now wouldn't try to
"click" any windows using DLGC_DEFPUSHBUTTON dialog code but which are not
really buttons -- but then this shouldn't ever happen anyhow.

include/wx/msw/window.h

@@ -14,6 +14,8 @@
 
 #include "wx/settings.h"        // solely for wxSystemColour
 
+class WXDLLIMPEXP_FWD_CORE wxButton;
+
 // if this is set to 1, we use deferred window sizing to reduce flicker when
 // resizing complicated window hierarchies, but this can in theory result in
 // different behaviour than the old code so we keep the possibility to use it
@@ -534,6 +536,16 @@ public:
     virtual wxMenu* MSWFindMenuFromHMENU(WXHMENU hMenu);
 #endif // wxUSE_MENUS && !__WXUNIVERSAL__
 
+    // Return the default button for the TLW containing this window or NULL if
+    // none.
+    static wxButton* MSWGetDefaultButtonFor(wxWindow* win);
+
+    // Simulate a click on the given button if it is non-null, enabled and
+    // shown.
+    //
+    // Return true if the button was clicked, false otherwise.
+    static bool MSWClickButtonIfPossible(wxButton* btn);
+
 protected:
     // this allows you to implement standard control borders without
     // repeating the code in different classes that are not derived from