Port preflight updater to Qt5

[eloquence]

As part of Qubes 4.1 readiness work (#600), we need to port the preflight updater to Qt5, as Qt4 is no longer supported on 4.1. This would be ideally behind a feature flag or detection logic, so we can land the change in main and easily switch over to it once 4.1 is out.

[eloquence]

As part of the next sprint, I'd like to do a first time-boxed investigation (possibly without using a Qubes 4.1 build for testing just yet).

[eloquence]

At first glance it looks like this should be relatively straightforward:

1. Due to some changes in how the PyQt module namespaces are organized, a couple of import statements have to be updated.

2. The existing Qt Designer UI can be imported into Qt Designer 5 without issues, and converted to code using pyuic5 without issues.

3. The generated UI (here in Ubuntu) appears to look and behave the same:

Now going to look into making a Qt5 version available via feature flag.

[eloquence]

Unless I've overlooked something major, #610 may resolve, or close to it; will do some testing in Qubes next week.