When using the notmuch backend, it often makes more sense to sort
folders (actual virtual folders, or queries) by the order specified in
the query-map file, rather than alphabetically. This patch introduces a
configuration option (disabled by default) that allows this.
Additionally, due to the notmuch backend previously using maps (which
are order-undefined) to store the list of queries, default query
selection on aerc startup fluctuated. This patch fixes that by using
slices to store query order.
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