This moves the UserAccounts panel to a page in the System panel.
This simplifies a lot of the existing code in the UserAccounts panel.
I did minimal changes to the sub dialogs so that those can be touched
in following changes, making it easier to review this one alone.
The main panel widget is now CcUsersPage, and is an AdwNavigationView
widget that has a default "current_user_page" page. Each page is a
CcUserPage (careful with the one-character difference between these
two classes).
Each CcUserPage has an associated ActUser object.
For GNOME 3.32 we are pushing to make all our avatar images
round. This commit adjusts the existing stock faces images
to avoid layout issues when they are cropped in this way.
Fixes#289
The faces that are included in the users panel are really old,
the style is outdated and they are too small for modern designs
as well as hidpi screens.
https://bugzilla.gnome.org/show_bug.cgi?id=792243
