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/textctrl.h

@@ -759,6 +759,8 @@ public:
         return GetCompositeControlsDefaultAttributes(variant);
     }
 
+    virtual const wxTextEntry* WXGetTextEntry() const wxOVERRIDE { return this; }
+
 protected:
     // Override wxEvtHandler method to check for a common problem of binding
     // wxEVT_TEXT_ENTER to a control without wxTE_PROCESS_ENTER style, which is