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/univ/combobox.h

@@ -161,6 +161,8 @@ public:
     virtual wxClientDataType GetClientDataType() const wxOVERRIDE;
     virtual void SetClientDataType(wxClientDataType clientDataItemsType) wxOVERRIDE;
 
+    virtual const wxTextEntry* WXGetTextEntry() const wxOVERRIDE { return this; }
+
 protected:
     virtual int DoInsertItems(const wxArrayStringsAdapter& items,
                               unsigned int pos,