Unresponsive window at the end of the initial phase

[Nathan Teodosio]

In the "first phase" of update-manager, the program updates the Apt cache and at the end of it calculates the updates for the system. The latter is done synchronously; It blocks the events in the Gtk window, including signals. That manifests itself to the end user as:

1. Inability to close the window via soft methods, such as INT or the window manager's close action. Xkill still works but leaves the process running.
2. Window stops being drawn and becomes just a passive box. E.g. when raising a window above update-manager and raising update-manager again, it just displays the contents of that other window.

That is captured in the attached video (umsl.mkv).

The operation (cache.saveDistUpgrade()) may take a long time to complete, but might be difficult to catch in systems reasonably up to date; Maybe that is why this wasn't reported as an annoyance before. My 'apt upgrade --dry-run' says 'Upgrading: 287, Installing: 1, Removing: 0, Not Upgrading: 180'.