py3: remove some Python 2 crufts
covers_lastfm: Fetch covers from Last.fm

The API key should be changed to something specific to Sonata.

It lacks currently any errors checking. It should at least do something if on
of the error code from http://www.last.fm/api/show?service=290 is returned.
Also, it seems sometimes Last.fm returns broken XML, so we should do
something about it.
covers_lastfm: Handle multiple images and errors checking
Port to Python3
covers-lastfm: make the plugin executable by finding some test cover
covers lastfm: new API key
covers: fix size of covers in the search remote covers dialog

For some reasons, it was working a long time ago, but didn't since a
while. I couldn't bisect the changeset, too many non-working changesets
during the Py3/Gtk3 migration branch :/

"162" is an arbitrary value, which is supposed to be the size of the
image plus the size of the "spiner" on the left.
Remove broken Rhapsody covers plugin
covers lastfm: port to JSON, bump to revision 1.0 (for what it's worth)
covers: use Last.fm services instead of (broken) Rhapsody to find covers

Merge branch 'covers-lastfm'