Add Desktop as a window to cycle with Alt+Tab

### Description of the new feature / enhancement

The ALT+TAB window cycling gives access to the desktop too.

WIN+D
WIN+M
(I know)

We need the ALT+TAB.

### Scenario when this would be used?

You are in a browser and want to cycle between the browser window and all your open windows, including the desktop.

### Supporting information

By searching the internet I found it is not just me! Many people need this feature and would find it very useful.

The issue names no files, tests, or entry points. Start by locating the existing Alt+Tab window-cycling implementation in PowerToys and determine how Windows represents the desktop in that flow. Define the expected cycling and focus behavior, then add coverage; done means Alt+Tab can include the desktop without breaking existing window cycling.

Written by the indexing model from the issue text.