Extract some BaseView methods into a static helper class.
Require contao-community-alliance/event-dispatcher ~1.3.
Introduce a session storage.

The session storage is environment related per definition. This solve
some issues with environment related session data, that are used across
environments.

This also deprecate the InputProvider::getPersistentValue(),
InputProvider::setPersistentValue() and
InputProvider::hasPersistentValue() methods.

The default session storage store its data in the user session.
Pass the action object to all view methods.
Extract BaseView::formatModel into an event.
Introduce view events.

A view event render a (partial) view. Contrary to actions, view events
simply render views. They should not contain complex business logic.
Start refactoring the clipboard system.

Improving the clipboard system, partially it is rewritten now, but it
needs a lot more work.
Add a view to render the clipboard.
Add a fixme.