RFE: issue change notifications for transient transaction elements too

[pmatilai]

Something that came up in discussion with the dnf team yesterday (@jrohel, @mblaha, @pkratoch):

Rpm issues change notifications for transactions elements as they get added and removed to the transaction to allow API users to keep track of the changes that rpm does behind the scenes (such as replacing an older version with a newer). The change notification is also handy for setting application private pointers to the elements, and at least dnf5 relies on this in their transaction callback. However, no notification is issued for the transient TE_RPMDB elements that are created by rpm for the sole purpose of giving a package context to trigger scriptlets whose owner is not part of the transaction but installed already.

We should issue change notifications for the create + destruction of TE_RPMDB elements too, but using some different event code from the normal elements because these are not being added/removed to the transaction as such, they are just temporary and short-lived elements.

[pmatilai]

A random note of the day: maybe we should actually remember these elements in the transaction, because they do take part in it, just not the usual added/removed manner.