a11y: Notify bad username

Sergio Costasrequested to merge
Sergio.Costas/gnome-initial-setup:fix-about-you-error-messages-a11y into ubuntu/latest

When the username is invalid, a text label is updated with the problem. Unfortunately, the content is not notified to the user.

This patch checks for changes in the label, and uses gtk_accessible_announce() to read out loud the text.

Forwarding: https://gitlab.gnome.org/GNOME/gnome-initial-setup/-/merge_requests/278
Bug-Ubuntu: https://bugs.launchpad.net/ubuntu/+source/gnome-initial-setup/+bug/2116935

Edited by Sergio Costas

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