Fork / maintainer change

[Qeole]

Pull Request before the add-on changes maintainer.

Summary for main differences:

• Compatibility with recent Thunderbird versions (38., 45.).
• Complete reorganization of the files within the repository.
• Complete reorganization of the packaging process (dropped the Rakefile, using a very simple Makefile instead).
• Drop support for unit test suite, at least for now. I could not have them work.
• Drop support for auto-update URL mechanism, at least for now.
• Drop support for Seamonkey suite, at least for now. Should be easy to add again if some Seamonkey user can test compatibility.

Minor differences include:

• Upgrad version number to 0.7.
• Fix a couple of bugs.
• Change default color theme.
• Add preferences for symbols for drawing tabs and spaces.
• Some file / code cleanup.

See individual commits for details.