blaplayer.py: rename audio sink

Signed-off-by: Niklas Koep <niklas.koep@gmail.com>
blaview.py: fix avoidance of a code path

Signed-off-by: Niklas Koep <niklas.koep@gmail.com>
Force download of cover when "Fetch cover" is used

Signed-off-by: Niklas Koep <niklas.koep@gmail.com>
Make position slider initially insensitive

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