feat: split flatpak update units into user and system

[castrojo]

This splits the updater service units into two, one for --user installed flatpaks and one for --system installed flatpaks.

The zenity thing defaults to --user but this should cover the case where users prefer --system. This also fixes #71

[castrojo]

Ok testing on a fresh install:

[jorge@fedora ~]$ systemctl status --user flatpak-user-update.timer
● flatpak-user-update.timer - Flatpak Automatic Update Trigger
     Loaded: loaded (/usr/lib/systemd/user/flatpak-user-update.timer; enabled; preset: disabled)
     Active: active (waiting) since Wed 2023-02-22 19:13:55 EST; 1min 24s ago
      Until: Wed 2023-02-22 19:13:55 EST; 1min 24s ago
    Trigger: Wed 2023-02-22 19:16:54 EST; 1min 34s left
   Triggers: ● flatpak-user-update.service
       Docs: man:flatpak(1)

Feb 22 19:13:55 fedora systemd[1424]: Started flatpak-user-update.timer - Flatpak Automatic Update Trigger.
[jorge@fedora ~]$ systemctl status flatpak-system-update.timer
● flatpak-system-update.timer - Flatpak Automatic Update Trigger
     Loaded: loaded (/usr/lib/systemd/system/flatpak-system-update.timer; enabled; preset: disabled)
     Active: active (waiting) since Wed 2023-02-22 19:12:02 EST; 3min 30s ago
      Until: Wed 2023-02-22 19:12:02 EST; 3min 30s ago
    Trigger: Wed 2023-02-22 19:16:54 EST; 1min 22s left
   Triggers: ● flatpak-system-update.service
       Docs: man:flatpak(1)

I'd prefer at least one more confirmation that we're doing it right. 🥷