Remove '%' string formatting in logging everywhere.

This changes have been applied to all python files using fix
logging_mod_operator from https://github.com/htgoebel/pep8ify (branch
coherence-project) and the result has been checked.
Make cursor right/left expand/collapse device tree view.
Cleanup: Move long if-branch into a method by it's own (part 1/2).

To keep changes small, indention levels are not corrected in this change.
Cleanup: Move long if-branch into a method by it's own (part 2/2).

This change is only fixing the indention levels. Nothing else changed.
Refactor: Move duplicate ugly code into a function.
Fix issue #219: Double Click should start default action.

When double clicking on an entry in the device list the submenu was
opened. For usability reasons this now starts the default-action
instead. So e.g. for a media server "browsing" is now opened directly.