Sync file list with file actions

Whenever file actions are registered later, now the file lists are
automatically notified.

Added FileActions.addUpdateListener() to be able to receive such
notifications.

This removes the need for apps to manually call FileActions.display()
after registering new actions.

This fixes issues with race conditions when file actions are
registered after the file list was already rendered.
Merge pull request #9254 from owncloud/fileactions-deferred

Sync file list with file actions