zypp Backend: With "minimal" upgrade, only download packages

This allows downloading all upgrades so that the upgrade can then
be carried out offline. Some preparations for other methods added.

[backends] zypp: Minimal system upgrade = download-only upgrade
zypp Backend: Keep track of overall upgrade/install progress

Count packages to be installed / removed before committing a
zypp transaction, so that we can update the overall upgrade
progress whenever a package installation / removal is finished.

[backends] zypp: Report overall upgrade progress
Merge pull request #6 from thp/download-only-on-minimal-upgrade

zypp Backend: With "minimal" upgrade, only download packages
Merge pull request #7 from thp/overall-upgrade-progress

zypp Backend: Keep track of overall upgrade/install progress