refactor/legacy_audio

[JarbasAl]

• helps keeping the legacy plugins alive independently of OCP
• use abstractmethods to force implementation of key pieces in plugins
• drop plugin dependencies on common_play base class -> port utils from https://github.com/OpenVoiceOS/ovos-ocp-audio-plugin/blob/dev/ovos_plugin_common_play/ocp/base.py#L66
• default playlist handling implementation, plugins don't need to worry about this and usage in ovos-audio is deprecated
• move methods around for readability (sorry for messing the git diff!)

companion PRs: https://github.com/OpenVoiceOS/ovos-audio-plugin-simple/pull/10 https://github.com/OpenVoiceOS/ovos-plugin-vlc/pull/7 https://github.com/OpenVoiceOS/ovos-audio/pull/64