Support protected system packages

[AsciiWolf]

I have already created a ticket for this here, however I am also creating a discussion thread since this feature in my opinion is extremely important for end-users.

GNOME Software / Snap Store / KDE Discover have support for system apps (and other packages) that are compulsory and are not uninstallable. When looking at the latest screenshot of the Flutter Software App, it looks like that all core packages/Snaps are actually user uninstallable. It is extremely important to implement a similar mechanism since it prevents users from removing system packages and breaking their system. The way it is implemented in GNOME Software / Snap Store / KDE Discover is by using the compulsory_for_desktop tag in AppStream metadata, however, that is not the only way how it could be implemented in the Flutter Software App.

For reference, this is how the protected packages look like in the gnome-software based Ubuntu Software on a clean Ubuntu 23.04 system:

And regular packages without AppStream metadata (for example most of the critical system packages) are not displayed at all.

This is how it looks like in the Flutter based Software app (latest 0.3.0-alpha from preview/edge Snap):

Notice that all the critical system packages/Snaps are there and I have actually confirmed that it is possible to uninstall them by clicking the "Remove" button.

It is also possible to uninstall the Software app itself:

A good start could probably be disabling removal of the snap-store app itself (and optionally also showing only the packages that contain AppStream metadata, see #1273). :-)