Use more interoperable format for DB instead of pickle

[jose1711]

I think pickle is not the best choice for storing ShortlistItem objects due to the following reasons:

• it's difficult to view/process pickled file externally (e. g. in a text editor)
• a compatibility issue may appear between different (Python/Kodi) versions
• xml format is typically used for user data in Kodi, other times sqllite or .json is used

[jose1711]

FWIW here's the way to dump shortlist file to terminal:

cd .kodi/addons/plugin.program.shortlist
python3
import pickle
items = pickle.load(open('/path/to/.kodi/userdata/addon_data/plugin.program.shortlist/shortlist.db', 'rb'))
for item in items: print(item.title, item.thumb, item.filename)