lib: Notify when installed database changed

[mcrha]

Similar to the "updates-changed" signal emit "installed-changed" when
the backend notifies about changes in the installed apps, thus the library
user can update its UI accordingly. The signal only means it's possible
something changed, it doesn't mean there changed anything.

In case of the libdnf backend this may require rpm-software-management/libdnf#1542 on some systems.

[mcatanzaro]

Ping reviewers

[mcrha]

For what it's worth, the libdnf counter part is merged now (rpm-software-management/libdnf#1542).

[ximion]

This needs documentation and maybe should be added to the backend compatibility matrix as well (otherwise I think this is a useful feature).

[mcrha]

This needs documentation

what kind of documentation do you mean, please?

maybe should be added to the backend compatibility matrix as well

Where is that, please? In the backends/dnf/README.md ?

[Conan-Kudo]

The documentation is here: https://github.com/PackageKit/PackageKit/blob/main/docs/html/pk-matrix.html

[mcrha]

Aha, I see. Thanks for the pointer.

[mcrha]

I added a commit with the added entry in the pk-matrix.html (there are two backends calling pk_backend_installed_db_changed()), though the file mentions methods, not signals, thus it might be confusing in some way.

[mcrha]

I updated the branch:

• applied Philip's suggestion
• rebased to the latest main branch