This allows us to open subpages of the "System" panel. The arguments/
parameters are expected to match the panel's subpage tags (as in
AdwNavigationPage.tag).
With this, a future change could reintroduce desktop files for the
subpages, and just change the Exec line to accomodate the new format.
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.
