header files: update installed header files

1. add VideoCommonDefs.h
2. do not install internal headers
3. move VideoEncoderDef.h to VideoEncoderDefs.h
Add vaapi into namespace.

Signed-off-by: Tang Xin <xin.t.tang@intel.com>
Add Capi wrapper controlled by enable-capi.

Signed-off-by: Tang Xin <xin.t.tang@intel.com>