blatracklist.py: use BlaMenu throughout

Signed-off-by: Niklas Koep <niklas.koep@gmail.com>
blaguiutil.py: use BlaMenu in favor of gtk.Menu

Signed-off-by: Niklas Koep <niklas.koep@gmail.com>
blatray.py: make use of BlaMenu methods

Signed-off-by: Niklas Koep <niklas.koep@gmail.com>
blatray.py: pass config in as explicit dependency

Signed-off-by: Niklas Koep <niklas.koep@gmail.com>
Break dependency between main window and tray icon

Signed-off-by: Niklas Koep <niklas.koep@gmail.com>
Let widgets call their own `show' methods

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