Fix #588 - Refactor plugins.

- Added settings.py with PluginConfig.
- Rewrote jackaudio's load_init()
That is much more concise. 

Related: is it possible to have a case where a higher device file exists and a lower one doesn't, so the loop exits too early? Like if /dev/fw1  is removed by the user when  /dev/fw2 was already in use for example.