Save and restore queue on shutdown

Signed-off-by: Niklas Koep <niklas.koep@gmail.com>
Focus active playlist on track changes

Signed-off-by: Niklas Koep <niklas.koep@gmail.com>
Fix bug when restoring queued URIs

Signed-off-by: Niklas Koep <niklas.koep@gmail.com>
Add decimal place to volume tooltips in dB

Signed-off-by: Niklas Koep <niklas.koep@gmail.com>
More PEP8 conformance changes

Signed-off-by: Niklas Koep <niklas.koep@gmail.com>