Marshal GValues that holds a STREAM FLAGS value

Change-Id: I8d507d94442f9f5925e674a012e9f96cdbd4643b
Marshal GValues that holds a FORMAT value

Change-Id: I4717c34bbf60a13ef3b157df6831f3a3d1be929f
Provide support for marshalling GstEvents

Change-Id: I0d7dca9498cfd3f7783875ca61a993ad44d1cddb