RFE: Provide pre- and post- hooks for uevent `add` and `remove` actions

[tbzatek]

Several times I came across the need to cross-update two objects interlinked together - e.g.

• Drive.Size property needs to be updated after the NVMe.Namespace interface uevent
• LogicalVolume needs to be updated after the VolumeGroup interface uevent
• btrfs/f2fs/bcachefs multidisk volumes will cross-link object paths

This is not a problem for add and change uevents as the required objects do still exist, however in case of a remove uevent the cross-linked objects will need to be notified that something has gone missing.