system: Add "Users" panel
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.