Add ws_raw_buffer type

This patch adds the (abstract) ws_raw_buffer type, which will be
the supertype of ws_image_buffer and ws_frame_buffer. The latter one
will be pulled up and placed directly below the raw buffer type.
Change inheritance for ws_image_buffer type

Inherit from ws_raw_image_buffer, remove all callbacks therefor.
Change ws_frame_buffer to inherit from ws_raw_image_buffer

This patch changes the ws_frame_buffer to inherit from
ws_raw_image_buffer instead of ws_image_buffer.

It therefor adds a buffer member to the type definition and implements a
get_data callback for it. It removes all other members, which are now
covered by the supertype.