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/srchctrl.h

@@ -82,6 +82,10 @@ public:
     virtual void SetDescriptiveText(const wxString& text) = 0;
     virtual wxString GetDescriptiveText() const = 0;
 
+#if wxUSE_NATIVE_SEARCH_CONTROL
+    virtual const wxTextEntry* WXGetTextEntry() const wxOVERRIDE { return this; }
+#endif // wxUSE_NATIVE_SEARCH_CONTROL
+
 private:
     // implement wxTextEntry pure virtual method
     virtual wxWindow *GetEditableWindow() wxOVERRIDE { return this; }