In some corner cases, an outgoing share exists but sharing is not
allowed for the current user. This would cause the file list to break
because the static text could not be rendered as the owner was
undefined.

This removes the logic that registers the share action and modifies the
rows. Share actions aren't needed in the public file list.

Whenever file actions are modified, either by registering new actions or
when appending a new page of entries, the sharing app is now notified so
it can correctly refresh the sharing icon status.

Additionally, the core's loadIcons() method is also used to load the
existing shares and also refresh the sharing icons afterwards.

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.

- scrolling to next page now correctly renders share icons
- reshareing/unsharing a reshare will now still keep "Shared by" label