validate: Print actions directly from the scenario

validate: Print actions directly from the scenario

Avoiding user to have to print them in each and every action type
implementation.

This requires adding some API to prepare actions before printing them.
Preparing action in that case mean parsing the values contained in the
GstStructure parsing equations and setting back the actual value
afterward

API:
  * GstValidatePrepateAction
  * gst_validate_action_type_set_prepare_function