[SinghCod3r]

When I click Forward or Apply in the New Printer wizard, the window sometimes freezes for a second or two. The cursor shows busy, nothing else happens, and then the next page appears. This PR fixes that.

I measured the click path and found two slow calls on the GTK thread:

• checking a driverless PPD
• creating the queue on Apply

What changed:

• Both calls now run on the existing CUPS worker (authconn run_on_worker), so the window stays responsive. Nothing is cut short, so slow printers just take as long as they need.
• One centered spinner shows right away and stays until the next page is ready.
• The bottom-left spinner is removed.
• The page is no longer dimmed while waiting. A clear click blocker is used instead.
• The Choose Driver page shows only its own loader.
• Back and Cancel keep working during a wait. A late result from a step you left is ignored.
• Apply is disabled while it runs, so a double click can't create the queue twice.
• Fixed a related bug: for a driverless printer whose PPD fails (the "Broken Xerox" case), the Makes list selected a "Broken" make that doesn't exist. Now it falls back to Generic with nothing recommended.

In this SMB is not touched, and I reused the existing worker and spinner code instead of adding new frameworks.

How I tested:

• test_newprinter.py and test_PhysicalDevice.py pass.
• Ran the GUI with Xerox alone, Broken Xerox alone, and both together (in both orders, using Back). No freeze, one spinner, and the Makes list is correct.