frontend/greeter: remember last session separately for each user++

[@genericity] frontend/greeter: remember last session separately for each user

The greeter now saves the last logged-in session separately for each user. When a user icon is clicked, the session dropdown automatically selects that user's last session (if available).

This improves the user experience when multiple users share the same device, and using different desktop environments or window managers. Or in my case, I created separate user accounts for Plasma release and dev builds.

The implementation is by replacing the single "lastLoggedInSession" string in the greeter state config, with a JSON string that maps usernames to their last sessions. Transistion from the old format is handled automatically.

frontend/greeter: Improve session button appearance and behavior

Use PC3.ComboBox, with flat styling, and a custom content item so the label elides rather than clips.

frontend/greeter: Don't time out greeter when session combobox popup is open

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Branched from !82 (closed) as I cannot add commits there.

Depends on libplasma!1557 (merged)