discovery: stop direct use of attribute of partialdiscovery
Instead of accessing undecided directly for ui display purposes,
we introduce a stats() method that could be extended in the future with
more interesting information.
This is in preparation for a forthcoming Rust version of this object.
Indeed, attributes and furthermore properties are a bit complicated for
classes in native code.
We could go further and rename undecided to mark it private, but _undecided
is already taken as support for _undecided lazyness.