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.

Fixes https://github.com/owncloud/core/issues/9026
Fixes https://github.com/owncloud/documents/issues/275

Please review @MorrisJobke @icewind1991 @VicDeo 

I'll send separate PRs for the music app and documents app to remove the bogus "FileActions.display()" call.