bladb.py: fix a few formatting issues

Signed-off-by: Niklas Koep <niklas.koep@gmail.com>
Move cell renderer constants out of blaguiutils.py

Signed-off-by: Niklas Koep <niklas.koep@gmail.com>
blabrowsers.py: rework library model creation

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