Search the themes the same way than plugins

Because it’s a pita to remember to do a symlink.

Changesets:
- Revision 64c1a090 by mathieui on 2013-08-09T22:24:27Z:

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