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/osx/combobox.h

@@ -102,6 +102,8 @@ class WXDLLIMPEXP_CORE wxComboBox :
 #endif // wxOSX_USE_COCOA
 
 
+    virtual const wxTextEntry* WXGetTextEntry() const wxOVERRIDE { return this; }
+
     // osx specific event handling common for all osx-ports
 
     virtual bool OSXHandleClicked(double timestampsec) wxOVERRIDE;