[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.

Added GajimPluginConfigDialog class - dialog that plugins should use to present configuration to user.
Now, 'Configure' button is invoked only for plug-ins that have config_dialog.

Added new 'init' method to Plugin class that plugins can implement to make actions that need to be done only once - when plugin is added (not activated) to Gajim. In this method plugins should declare handlers for GUI extension points. This was created so that __init__ method doesn't have to be reimplemented in specific way (create config, load config) - it is all done by __init__ in Plugin class. If  __init__ is reimplemented, it must call Plugin __init__ (eg. using super() ) to plugin work properly.

Example plug-ins were modified to use init() instead of __init__().

Added new category in configuration - 'plugins'. It only holds one option for each plugin - 'active', which determines whether plugin should be activated on startup.

Now, Gajim remembers which plugins are active on exit, and activates them on next startup.

Added 'chat_control_base' GUI extension point (now we are able to modify all chat controls - both normal and GC.

Added base version of AcronymsExpanderPlugin with some hardcoded acronyms. It uses newly added 'chat_control_base' extension point.