system: Add new Sytem panel

This adds the new System panel, result of the work of @gautham_x during GSoC 2023 with some changes made by me.

Mockups at https://gitlab.gnome.org/Teams/Design/settings-mockups/-/blob/master/system/system-panel.png

This is still missing the Users page, which I am rewriting separately to fix some longstanding issues with our implementation (it won't be a simple move from panels/user-accounts to panels/system/users/ as the others).

See #2241 (closed)