Base class for plugins. A plugin MUST NOT have any side effects when it is instantiated. This is necessary so that it can be safely loaded by unit tests, and so that a user interface can allow the user to disable it, even if it is installed, with no ill effects. Any side effects that would normally happen should occur in the enable() method, and be undone by the disable() method. These methods must be callable any number of times. The subclass MAY define the following attributes: * name * description * version * required_application_version name is the user-visible identifier for the plugin. It defaults to the plugin's classname. description is the user-visible description of the plugin. It may be arbitrarily long, and can use pango markup language. Defaults to the empty string. version is the plugin version. Defaults to '0.0.0'. It MUST be a sequence of integers separated by periods. If several plugins with the same name are found, the newest version is used. Versions are compared integer by integer, starting with the first one, and a missing integer treated as a zero. If two plugins have the same version, either might be used. required_application_version gives the version of the minimal application version the plugin is written for. The first integer must match exactly: if the application is version 2.3.4, the plugin's required_application_version must be at least 2 and at most 2.3.4 to be loaded. Defaults to 0.