Backend support for "Pinning" functionality of apps in sub-menus

[ninavizz]

This is a child issue within the epic #6665

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The problem you're addressing (if any)
For #5677 many users suggested the notion of enabling folks to "pin" their favorite apps to the top of a qube's sub-menu in a new, qubes-specific app menu. Just as owners of a GitHub repo may pin issues to the top of their issues page, or in Slack users may pin posts to the top of a channel.

Describe the solution you'd like
#6671 was created to surface "pinning" functionality in the GUI. The backend functionality, however, still needs to be created.

Where is the value to a user, and who might that user be?
Making it easier for users to quickly find what they need in their app menus.

Describe alternatives you've considered
Today's app menu experience. Lots. Of. Scrolling. Visual scanning.

Additional context
Functionality has been demonstrated in mockups for #6665 and in the videos created for Bessie and Jackie prototypes.

Related, non-duplicate issues
Cited inline, above.

[ninavizz]

@andrewdavidwong @marmarta It just occurred to me, that if the Settings in 4.1 are already fixed to the top of submenus in the app menu, that this backend functionality may already exist?