ENH: Re-introduce support for custom logic handling VR complex gesture
This commit re-introduce the support for overriding RecognizeComplexGesture()
removed in af0fab48 (Clean up VR and OpenVR classes) and originally introduced in f02a9b42 (COMP: Make function in OpenVRRenderWindow virtual)
This is intended to address https://github.com/KitwareMedical/SlicerVirtualReality/issues/91