plugin download: use cache if dest_path is empty: if dest_path is not set, the file will be downloaded in cache. A new name will be generated using a hash of the uri. If the file is already downloded in cache, it won't be downloaded again (in this case returned progress_id is an empty string).

# XXX: the Monkey Patch is here and not in src/__init__ to avoir issues with pyjamas compilation
# TODO: remove this when changes are merged in Wokkel
from sat_tmp.wokkel import install
install()