[Dicson] use manifest.ini files in plugins to have a separate file for plugin's information. Fixes #5920

Also use sed to remove now unneeded Vim lines, 2to3 -f ws_comma to fix
some whitespace, and fix some other madness manually.

Improvements to GUI extension points handling - added method to remove these from PluginManager (memory optimization).

Removed logging from most of the code.

Length Notifier Plugin has configuration dialog (added entry with JIDs to be included when plugin is working) - fully usable.

Default values of configuration key has been added to GajimPlugin.

Some other minor changes/fixes.