Decouple the app from its plugins

At the moment, plugins rely on the app for their components (panels, widgets, etc.). It may be better to refactor shared components/utilities to a separate package.

To this end, I've developed [aiidalab-qe-base](https://github.com/edan-bainglass/aiidalab-qe-base). So far, I've tested the app and its built-in plugins with all imports pointing to this repo.

Happy to get some feedback on the idea and implementation. Note that there are still a few things to migrate, but this should be done shortly. You can already have a go at it.

Pinging...
@superstar54
@AndresOrtegaGuerrero
@mikibonacci