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`
validateflow can be used to check the buffers and events flowing through
a custom pipeline match an expectation file. This can be used to test
non-regular-playback use cases like demuxers handling adaptive streaming
fragment pushing.
This patch includes also new actions used for these cases:
`appsrc-push`, `appsrc-eos` and `flush` (plus `checkpoint`, which is
only available with validateflow).
