core/video: separate PyGST / Gi Gstreamer video providers in 2 separate modules. refs #1689
audio: implement a separate Gi-based audio provider, and automatically deprecate Pygst if gi is find.
camera: same as audio, add a core provider based on Gi.
audio/video: few import fixes
core/gi: dont use Gi bindings for older gstreamer (< 1.0)
gi/doc: add entry in the FAQ about gstreamer/gi, and reference it in video/audio/camera core provider.
gi: fix typo
Merge pull request #1692 from kivy/fix-1689

Separate PyGST / Gi Gstreamer