ENH: Re-introduce support for custom logic handling VR complex gesture

This commit re-introduces 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)

Since the original RecognizeComplexGesture() was also re-factored by
introducing HandleGripEvents() now responsible to handle the start and end
of the complex gesture, this commit also make HandleGripEvents() virtual.

Co-authored-by: Lucas Gandel <lucas.gandel@kitware.com>