[Feature] Integration of pacman-offline

[rkunschke]

Pacman offline is using systemd-offline-updates to perform automatic updates on arch. It downloads (prepaires) updates while the system is running and installs them at shutdown process.

It would be nice to have some integration to the arch-update checker since pacman-offline does not provide any feedback to the user while preparing an offline update.

Basic features:

• provide a info about the number of prepaired updates via pacman-offline
• mark the prepaired updates in the package list

Advanced features:

• trigger prepairing updates
• -> trigger the pacman-offline prepaire service everytime arch-update-checker is looking for updates
• track progress of prepairing updates
• warn if also AUR packages need updates since they are not included in pacman-offline