Generic snap utilities should be moved to a common source package for everyone to share

[Simon Quigley]

The Lubuntu team wrote two snap-based utilities that we would like to move to a common source package in the OO cycle, perhaps snapd itself. These are snap-preseed-glue and simple-snap-preseed-dialog, both of which serve distinct purposes:
 - snap-seed-glue is a command-line tool which allows for preseeding snaps. We use this in Calamares so users can select Krita, Thunderbird, and/or Element via the UI to be installed as a snap on the target system. It does not clobber existing preseeded snaps (in our case, Firefox and Firmware Updater), but instead adds the appropriate snaps (and their default content snaps, recursively). seed.yaml items are treated as a set. This should eventually be used in livecd-rootfs.
 - simple-snap-preseed-dialog is a simple GUI application meant to be ran on system startup. It lets the user know that the preseed process is still running in the background, so they are not left with a bad impression if they try to use e.g. Firefox right after boot, but it is not fully installed yet. It is incredibly simple, and uses 3.2 MB of RAM (precisely) when it needs to do something, 2.1 MB (precisely) of RAM if ran on startup without functionality.

snap-seed-glue currently exists in calamares-settings-ubuntu, is written in Go, and needs golang-github-snapcore-snapd-dev as a build dependency.

simple-snap-preseed-dialog currently exists as a dedicated binary package of lubuntu-default-settings, and is written in Qt 5 + C++. We named the binary package as lubuntu-snap-installation-monitor in lieu of an agreed-upon, generic name for it, to minimize upgrade conflicts in the future. This should likely be available as both a Qt and GTK frontend Eventually, to allow GTK-based desktop environments to use this functionality as well, without the extra Qt runtime dependencies.