Sub-actions were really hard to use and conceptually weird. The
implementation was ugly and made the code complex for nothing.
Instead this commit introduces a `foreach` action type which allows
repeating actions passed in an `actions` array the number of time
specified by any `GstIntRange` value defined in the structure or its
`repeat` field.
This commit also makes sure that all action got through
gst_validate_action_set_done upon finalization.
+ Cleanup surrounding code
+ Add tests
Part-of: <https://gitlab.freedesktop.org/gstreamer/gst-devtools/-/merge_requests/207>
