NodeListener::NodeChanged(): Provide old value

* Add interface OldNodeAttributes an instance of which is passed to
NodeListener::NodeChanged() to provide the old attribute values
(currently only modification time and file size).
* Also extend PackageLinksListener::PackageLinkNodeChanged() with a
OldNodeAttributes parameter.
* Add OldNodeAttributes implementations for PackageLinkSymlink (inner
class OldAttributes) and UnpackingNode (OldUnpackageNodeAttributes).

Fix package link updates/node monitoring

* PackageLinksListener: Rename methods from *Directory*() to *Node*()
and change parameter to Node*. Also add a *Changed() method.
* PackageLinkDirectory: Update only when necessary and what is necessary
(currently only the self link).

Add package link directories to system volume

* Add PackageLinksListener interface. It is used by
PackageLink[s]Directory to notify a listener about changes.
* Volume does now implement PackageLinksListener and recursively adds/
removes package link directories.

This makes the package link directories appear at least. They are still
empty.

NodeListener::NodeChanged(): Provide old value

* Add interface OldNodeAttributes an instance of which is passed to
NodeListener::NodeChanged() to provide the old attribute values
(currently only modification time and file size).
* Also extend PackageLinksListener::PackageLinkNodeChanged() with a
OldNodeAttributes parameter.
* Add OldNodeAttributes implementations for PackageLinkSymlink (inner
class OldAttributes) and UnpackingNode (OldUnpackageNodeAttributes).

Fix package link updates/node monitoring

* PackageLinksListener: Rename methods from *Directory*() to *Node*()
and change parameter to Node*. Also add a *Changed() method.
* PackageLinkDirectory: Update only when necessary and what is necessary
(currently only the self link).

Add package link directories to system volume

* Add PackageLinksListener interface. It is used by
PackageLink[s]Directory to notify a listener about changes.
* Volume does now implement PackageLinksListener and recursively adds/
removes package link directories.

This makes the package link directories appear at least. They are still
empty.