Need to rethink "config needed" indications now that we're multibackend

Now that Encuentro is multibackend, we don't need usr/pass for all backends, so this changes some things regarding configuration and download availability:

- We need a "have_config" for *each* backend (those who don't need anything will always return True, those that need something (i.e.: usr/pass) will return True only if have that.

- The "download" button in toolbar/menu will not longer be enabled generically, but everytime an episode is selected, accordingly to the have_config() of its backend

- The "need something indicator", for configs, will check also all(x.have_config() for x in all_backends)

- The "Download" in right-button menu need to check on have_config() for its backend