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/msw/combobox.h

@@ -128,6 +128,8 @@ public:
 
     virtual void SetLayoutDirection(wxLayoutDirection dir) wxOVERRIDE;
 
+    virtual const wxTextEntry* WXGetTextEntry() const wxOVERRIDE { return this; }
+
 protected:
 #if wxUSE_TOOLTIPS
     virtual void DoSetToolTip(wxToolTip *tip) wxOVERRIDE;