Implement wxTextCtr SetSelection and GetValue methods (needed by SetValue) in wxQt
Derive wxQtSpinBox polymorphy helpers from wxQtEventSignalHandler to proccess QSpinBox events. Note that wxQtSpinCtrlBase is not a helper derived from Qt (it is in fact the wxQT implementation derived from wxSpinCtrlBase)
Implement spin control event in wxQT (connecting the value changed signal, similar to wxGTK)