Class ModeBasedActiveMixin

Checks we are active.

This method will be called with the session to check if this specific class is active. This mechanism allows multiple implementations to all share the same name, as long as only one is actually active. For example, we can have a linux, windows and mac version of plugins with the "pslist" name.

This mixin provides the mixed class with a basic is_active() method which honors a mode member defined on the class and all its subclasses. The mode is additive (meaning each class and its subclasses are only active if the mode is active).