The picker sorted folders A-Z by path, with Inbox first, so a folder
dragged into place in the sidebar turned up somewhere else when moving
mail. It now walks the tree in compareFolders order, the sidebar's
order with every folder expanded: Inbox, then the saved order, then
the special folders, then A-Z, with subfolders under their parent.
treeOrder lives beside compareFolders. A folder the walk from the top
cannot reach is appended rather than dropped, so it stays pickable as
it was before.
Closes#1