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/gtk1/combobox.h

@@ -157,6 +157,8 @@ public:
     static wxVisualAttributes
     GetClassDefaultAttributes(wxWindowVariant variant = wxWINDOW_VARIANT_NORMAL);
 
+    virtual const wxTextEntry* WXGetTextEntry() const { return this; }
+
 protected:
     virtual int DoInsertItems(const wxArrayStringsAdapter& items,
                               unsigned int pos,