feat: update mode

[NN708]

Resolves #410.

This PR adds an update mode, which is automatically activated when ~/.local/share/vanilla-first-setup/seen_app_ids.json exists.

In update mode, the user is prompted to:

• Install any apps listed in apps.json install lists, that are not in the seen_app_ids.json install list, and not already installed.
• Uninstall any apps listed in apps.json deprecated list, that are not in the seen_app_ids.json uninstall list, and are already installed.

The SHA-256 hash of apps.json is also stored in seen_app_ids.json, so First Setup runs only when apps.json has changed.

[taukakao]

How are we gonna make sure that first-setup runs on login when the startup file was deleted for most users?

Maybe we should rather use a user systemd service in /usr/lib to run first-setup?

[NN708]

How are we gonna make sure that first-setup runs on login when the startup file was deleted for most users?

Maybe we should rather use a user systemd service in /usr/lib to run first-setup?

Thanks for the suggestion! You're right that a system-wide approach would be more robust than a per-user one. But since first-setup is a desktop app, placing a .desktop file in /etc/xdg/autostart is the more conventional and simpler approach. I've added it there, and verified in a VM that it works as expected.

[taukakao]

I think both are about as conventional by now but the xdg folder is fine by me.