Clean up inheritance mess for integrations

[MaddyGuthridge]

Currently, integrations are a mess - it's immensely difficult to get a solid mental model of them.

In order to address this, some major refactoring should be done:

• Have the one Integration base class which all integrations inherit from
• Simplify the interface such that integrations are the same for PluginIntegration, WindowIntegration and CoreIntegration
• Move CoreIntegration's shouldBeActive classmethod to be part of the base class definition, and should probably be called isEnabled or something
• Simplify the extension manager system to allow for integrations to be registered alongside their activity criteria (ie use a decorator that allows users to specify the conditions for its activity)