Move GstPlanarAudioAdapter to -base

CC @gkiagia

Open questions/issues (please add more)

• Should it also work for interleaved audio? IMHO yes but it currently doesn't, we would need a different name then (GstAudioAdapter)
• GstMapFlags usage is weird: if no write-mapping is requested this does only shallow copies of buffers. But always doing that would simply cause the deep-copy of the memory to happen on write-mapping so that's exactly the same overall. What is the rationale here?
• Use a GQueue or other more optimal data structure (g_slist_append() and counting list length manually is not great)

Otherwise looks fine to me at least.