validate:flow: Add a way to set the types of events to log/ignore

validate:flow: Add a way to set the types of events to log/ignore

Added two properties to the plugin:
     * ignored-event-types: A list of event types to be ignored when logging events
     * logged-event-types: A list of event types to be logged when logging events

This commits also moves the "ignored-event-fields" property to using a proper
GstValueList for the list of event fields to be taken into account, instead
of the home grown separated by comas list of string, making the API more
uniform.

This also adds a simple helper method: `gst_validate_utils_get_strv`