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.
Adding the 'error' css class to icons visually indicates that
there's an issue with the input. This is the same purpose served
by the status icons. Another reason to remove the status icons
is that with the error class added to entries the entries dim
except for the status icons.
Currently, the password_entry_timeout() callback can be called even after
the dialog disposal, which leads to segfaults. This is because the
corresponding source ids can be cleared in some cases without removing
the sources. Let's prevent direct calls of password_entry_timeout()
without destroying the source ids in order to fix this issue.
Fixes: https://gitlab.gnome.org/GNOME/gnome-control-center/issues/760
