ofVideoGrabber: return a vector of ofVideoDevice

don't merge yet, working for gstreamer only

i've added 2 new classes ofVideoDevice and ofVideoFormat. probably not all the fields will be able in every platform and perhaps some others make sense in osx or windows

the videoGrabbers will fail now compiling since they don't implement the new version of listDevices so we should fix android and iphone even if they don't return anything