This allows to save JavaScript plugins, files with .js extension,
into `$PREFIX/share/evolution/webkit/preview-plugins/` or into
`~/.local/share/evolution/preview-plugins/` for the message preview
and into `$PREFIX/share/evolution/webkit/webkit-editor-plugins/` or
into `~/.local/share/evolution/webkit-editor-plugins/` for
the WebKit composer.
The plugin is an object, which contains two members:
* `name` - a string with a name of the plugin
* `setup(doc)` - a setup function
The argument of the `setup` function is a `document`, which can
be either the main document or an iframe document. The function
can be called multiple times for the same document.
The preview plugin registers itself by calling `Evo.RegisterPlugin(plugin_instance)`.
The WebKit editor plugin registers itself by calling `EvoEditor.RegisterPlugin(plugin_instance)`.
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