This also fixes the ".cutout-button" style being
set in "user-accounts-dialog.css", which is wrong
because this widget is located in the main panel
and not in the add user dialog.
When the Users panel got moved to System, the original .desktop file was
also moved. However, that is now obsolute with
`panels/system/users/gnome-users-panel.desktop.in`. This old file is
also not referenced in any `meson.build` and contains an outdated `Exec`
line.
So, remove it.
The implementation is based on the cc-add-user-dialog.c codes, but it is
not a pure copy&paste. It has to be changed to fit the skeleton and reflect
changes from the mockup. I've also tried to simplify the code where possible.
It also fixes some memory leaks.
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.
