With the settings redesign enabled, a session-restore restart sometimes
leaves about:preferences#home partially rendered.
The newtab AboutPreferences feed registers the Home pane's "homepage",
"customHomepage" and "home" groups when it observes "home-pane-loaded".
If a restored `#home` tab loads first, the one-shot notification fires
unobserved and the groups never register. SettingPane.init() then throws
`Setting group "homepage" not found` and aborts the rest of Home pane
init. The pane is already marked initialized by then, so only a full
restart recovers it.
Init now skips unregistered groups instead of throwing.
SettingGroupManager gains a `has()` check and an `onRegister()` listener.
Each setting-group subscribes and renders once its group registers.
Differential Revision: https://phabricator.services.mozilla.com/D310885
Tim Giles
b958d8d335