gst: encode: add property help functions for encoder properties.

gst: encode: add property help functions for encoder properties.

The encoder is a true gstobject now and all the properties are using
gobject's properties mechanism. Add help functions to handle the properties
between encode and encoder class.
The basic idea is mapping the same property between encoder and encode. All
the encoder's properties will have the same name, the same type in encode.
The set/get property function just forward the property setting/getting to
the encoder using the same property name and value. Because the encoder is
created on needed, we need to cache the property setting in encode.