Deezerart: Use astrcmp and make matching threshold configurable

[phw]

The deezerart plugin used Python's difflib for string similarity comparison. This has some problems:

1. difflib ist not part of the Picard binary packages for macOS and Windows, as the bundled Python only includes those modules actually used

2. The SequenceMatcher.quick_ratio only compares length and characters, not sequences (which means order of characters does not matter). See for example:

   >>> from difflib import SequenceMatcher
   >>> SequenceMatcher(None, "michael jackson", "joachim slacken").quick_ratio()
   1.0
   >>> from picard.util.astrcmp import astrcmp
   >>> astrcmp("michael jackson", "joachim slacken")
   0.3333333134651184
   

Use Picard's astrcmp function instead. A value of 0.6 seemed sensible in my tests. But a configuration option was added so users can tweak the threshold if they get bad results.

[zas]

LGTM