Split up offline update downloading and installing
This implements a part of the autoupdates mockup that commit 9df8f0a6 missed, adding an explicit Download step before the Restart & Update for offline updates.
If we don't do that, then we get a super confusing user experience where clicking on "Restart & Update" takes several minutes (or possibly several hours on slow network) and the page is seemingly hung, without any feedback, and then minutes or hours later out of the blue we suddenly get the shell restart dialog (which has a 60 second timer, rebooting when it expires).
It's a rather large change, but I believe we need to do it for 3.30, otherwise it's just too confusing. The MR doesn't add any new strings and reuses an already translated "_Download" string from the distro upgrades banner.
The MR is for 3.30, but I'll of course pick it to master as well.