Device playlists & two-way sync
Style cleanups: Trailing whitespace, "is None"

In src/gpodder/gtkui/main.py: Use "callback is None"
instead of "callback==None", and also make the function
signature nicer (spacing, newline).

Remove trailing whitespace in all files affected by the
latest device sync patch:

  git show HEAD | diffstat -l | xargs sed -i 's/\s\+$//g'