Add Plugin-System

[Narigo]

Is your feature request related to a problem? Please describe.
The currently available script-editor is not able to hook into events like "a todo changed" or "a new todo was added" etc. It would be great to listen to events, hook custom logic to it and update things according to it.

Describe the solution you'd like
A plugin system so I can install some kind of script that hook into changes of the todo tree.

Describe alternatives you've considered
Creating extensions in other ways or always running a script after changing something manually would not be user-friendly and is probably not flexible enough - especially when you want to change something due to some user change.

Additional context
It would be nice to "install" the plugins, so they will be always available on the account.