videoaggregator: Expose vmethods to set converters and prepare/clean frames

videoaggregator: Expose vmethods to set converters and prepare/clean frames

This gives more flexibility to the subclasses and permits to remove the
GstVideoAggregatorClass->disable_frame_conversion ugly API.

WARNING: This breaks the API as it removes the disable_frame_conversion
field

API:
  + GstVideoAggregatorClass->find_best_format
  + GstVideoAggregatorPadClass->set_format
  + GstVideoAggregatorPadClass->prepare_frame
  + GstVideoAggregatorPadClass->clean_frame

  - GstVideoAggregatorClass->disable_frame_conversion

https://bugzilla.gnome.org/show_bug.cgi?id=740768