Replace dynamic casts with virtual wxWindow::WXGetTextEntry()

Instead of checking for all text-like controls one by one in
wxCommandEvent::GetString(), call a virtual function checking for this.

This is simpler, less error-prone and faster -- at the cost of
increasing the vtbl size of all wxWindow-derived classes.

Closes https://github.com/wxWidgets/wxWidgets/pull/1696

include/wx/motif/combobox.h

@@ -108,6 +108,8 @@ public:
     virtual void Popup() { wxFAIL_MSG( wxT("Not implemented") ); }
     virtual void Dismiss() { wxFAIL_MSG( wxT("Not implemented") ); }
 
+    virtual const wxTextEntry* WXGetTextEntry() const { return this; }
+
 protected:
     virtual wxSize DoGetBestSize() const;
     virtual void DoSetSize(int x, int y,