userWidget: Allow vertical orientation for user avatars

userWidget: Allow vertical orientation for user avatars

Allow vertical orientation for the userWidget so that the user-avatar
can be centered and user's name can be placed below it. The plan
for 3.36 is to use this vertical userWidget layout for both lock
and login screen.

The userWidget is also used while creating the user-selection list
at the login, hence we still need to keep the horizontal layout
for userWidget in place.

https://gitlab.gnome.org/GNOME/gnome-shell/merge_requests/922