accountmodel: improve handling for account list reordering

This update fixes an issue where a stale account list might be used
in AccountListModel by synchronizing list invalidation with a newly
introduced signal (accountsReordered). This change prevents a data race
that could occur due to the asynchronous nature of setting the account
order over D-Bus, or when queuing the signal handling for
accountsChanged, which is emitted only once the reordering is saved.

As a result, QAbstractItemModel mutations are now performed within AccountListModel instead of in the UI.

Gitlab: #1638 (Account list in popup is incorrect after selection)
Change-Id: I7ed6eeb45eb319f21e40554f3d023ad24e139a6f