Add wxTextEntry::ClickDefaultButtonIfPossible() to wxMSW

Factor out this function from wxTextCtrl to allow its reuse in
wxComboBox in the upcoming commit.

No real changes, this is a pure refactoring.

include/wx/msw/textentry.h

@@ -81,6 +81,10 @@ protected:
     virtual bool DoAutoCompleteCustom(wxTextCompleter *completer) wxOVERRIDE;
 #endif // wxUSE_OLE
 
+    // Helper for wxTE_PROCESS_ENTER handling: activates the default button in
+    // the dialog containing this control if any.
+    bool ClickDefaultButtonIfPossible();
+
 private:
     // implement this to return the HWND of the EDIT control
     virtual WXHWND GetEditHWND() const = 0;