GstMfx: Replace deprecated apis.
GstMfxEnc: Set GopPicSize = 24 (key-int-max).
GstMfx: Update code layout.

1. Add base class: GstMfxBase.
2. GstMfxEnc/Dec/Scl based on GstMfxBase.
3. Using GstDebug replace of GDebug.
GstMfxBase: Add properties.

impl: MFX implentation.
version: MFX version.
async-depth: MFX async queue depth.
protected: MFX protected.
io-pattern; MFX IO pattern.
GstMfxBase/Enc/Scl: Fix mfx video context close.
GstMfxScl: Fix waiting for pop exec task.
GstMfxScl: Fix mfx session, using parent's instance.
GstMfxBase: Fix property type (GEnum -> GFlags).
GstMfxBase/Enc/Scl: Reuse base class properties.
GstMfxEnc: Export mfx encoder params by props.