(but since it is not important, use distutils when setuptools is
unavailable)

also remove old stuff from the default config

(no idea why this is appearing now)

- 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

(use new default directory, install the plugins as a separate
python module…)