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.
Since OC.Share didn't have any array containing the list of shares for
the current file, OC.Share.currentShares has been introduced to contain
the full share item structure instead of the reduced one
OC.Share.itemShares.
The event "sharesChanged" is now passing OC.Share.currentShares, which
itself includes the display name to be displayed for the recipients in
the action icon.
