Do not do backslash substitution on regex queries

As discussed in bug beetbox#3867, backslash replacement in query strings is a bit of a
hack but it is useful (see beetbox#3566 and beetbox#3567 for more discussion). However,
it breaks many regular expressions so this patch stops the replacement if the
query term contains '::', indicating it is a regex match.

This commit fixes beetbox#3867.

Signed-off-by: Graham R. Cobb <g+beets@cobb.uk.net>

beetsplug/web/__init__.py

@@ -244,7 +244,9 @@ class QueryConverter(PathConverter):
 
     def to_python(self, value):
         queries = value.split('/')
-        return [query.replace('\\', os.sep) for query in queries]
+        """Do not do path substitution on regex value tests"""
+        return [query if '::' in query else query.replace('\\', os.sep)
+                for query in queries]
 
     def to_url(self, value):
         return ','.join([v.replace(os.sep, '\\') for v in value])