Implement support for Ctrl+Backspace in plain EDIT controls

This shortcut is undocumented, but works in rich edit controls and even
in plain ones if they use SHAutoComplete(), so support it in all
controls because this is what people expect.

include/wx/msw/textctrl.h

@@ -294,6 +294,13 @@ private:
     virtual void MSWProcessSpecialKey(wxKeyEvent& event) wxOVERRIDE;
 #endif // wxUSE_OLE
 
+    // Do we need to handle Ctrl+Backspace ourselves?
+    bool MSWNeedsToHandleCtrlBackspace() const;
+
+    // Delete backwards until the start of the previous word before caret in a
+    // way compatible with the standard MSW Ctrl+Backspace shortcut.
+    void MSWDeleteWordBack();
+
     void OnKeyDown(wxKeyEvent& event);
 
     // Used by EN_MAXTEXT handler to increase the size limit (will do nothing