Move the most relevant options to the top of the default config file

(people may not want to read the whole file to setup their account)
Fix #2337 (search the themes the same way than plugins)

- Load the themes from:
    1 - The sources found in the directory ../data/themes/ (if it
    exists)
    2 - The user-defined dir (or ~/.local/blah)
    3 - The poezio_themes package if found

- Also fix some potential issues with the plugins importer
Fix #2343
Fix potential issues with import paths
Forgot the __init__.py for the poezio_themes package
Typo
Mark OTR messages as private (as in carbons)
Remove debug
Move shlex to poezio_shlex to avoid conflicts with the stdlib
Fix an issue with shlex

(well, sort of fix)
quote a completed word if it has \ in it
Fix /otr fpr and the remote user nick color