DeviceManager: Remember previously selected devices

[bluesabre]

This branch updates the device manager to remember previously selected devices and keep them available. This helps users with advanced audio setups, removing the need to return to the Sound plug any time they reconnect. Please let me know if you'd me to make any changes, thanks!

Fixes #189

[tintou]

I first thought that this should be the role of PulseAudio itself to do this, but this is actually a good practical way to achieve this result. The implementation looks good, my only complain would be that there is no way a device would be removed from preferred-devices, maybe it'd be useful to keep a last-used timestamp and remove it if it's been unused since a week or two (or some reasonable time), because if you select the wrong device then you're stuck with it 🙂

[bluesabre]

I first thought that this should be the role of PulseAudio itself to do this, but this is actually a good practical way to achieve this result. The implementation looks good, my only complain would be that there is no way a device would be removed from preferred-devices, maybe it'd be useful to keep a last-used timestamp and remove it if it's been unused since a week or two (or some reasonable time), because if you select the wrong device then you're stuck with it

@tintou Thanks for the review. I've implemented your suggestion of maintaining a timestamp with each preferred device, automatically removing devices that go unused for 7 days. Please let me know if you have any other changes.

[danirabbit]

This seems to work as intended. Nice job :)