Core component decoupling

[bartfeenstra]

Proposals

• Split extensions into app extensions and project extensions
• Make Project depend on App instead of vice versa, so we can have multiple projects open per app
• In the CLI API, merge @app_command and @global_command, and inject the app into any and every command. We may need a @project_command though.
• Add an interface for core components that allows them to be started or stopped. Because App, Project, and Extension all need resource management for their services.
• Add a method to (some?) component interface for registering event handlers. Seeing as handlers can only be set when a dispatcher is present, this means projects can only register with apps, and extensions should only register with projects. Therefore we need an unregister method as .
• Each component has a boot() and shutdown() method. These are called automatically when entering the component as a context.
• Refactor extensions into project extension plugins.

Blocked by

• The 0.4.x branch being opened
• https://github.com/bartfeenstra/betty/issues/980
• https://github.com/bartfeenstra/betty/issues/1537