Use class attributes where useful

Instead of using instance attributes for everything, this converts
to class attributes where useful.
Remove unused import of mpdhelper

Library classes access it via reference from main, so
the import is not used.
Add placeholder class attributes in LibraryView
Use descriptive variable names

This doesn't touch the bd/wd conventions, as at least wd is mirrored
in the config module.  If we change it, it should change there too.