Delete the remaining usages of BeetsPlugin.listen().
Since BeetsPlugin.listeners are wrapped by a loglevel-setting function,
we cannot easily check their unicity anymore.
BeetsPlugin._raw_listeners set holds the raw listeners.
Legacy plugins that did not handle enough arguments in their listenings
functions may break: dedicated code is now deleted for it would not work
with the decorated listeners.
Tests got fixed. Some modifications were done empirically: if it passes
then it's okay.
Include import of __future__ features division, absolute_imports and
print_function everywhere. Don't add unicode_literals yet for it is
harder to convert.
Goal is smoothing the transition to python 3.
