implement a foldable tree for the directory list. Expand all parent
directories when a hidden directory is selected with the change-folder
command.
folders-sort considers the top-level directories only. The folders and
foldersexclude filters work with the full directory path.
Enable tree view by adding 'dirlist-tree=true' to the config file.
Implements: https://todo.sr.ht/~sircmpwn/aerc2/228
Signed-off-by: Koni Marti <koni.marti@gmail.com>
