Manage multiple folders for a source

Currently a source (ie, a connection to a mail account - Server - or reading from a file in mbox or other formats - Stores), is defined by its connection parameters, and a specific folder to read emails from.

It should be possible to define multiple folders to read email from, for a same source, to avoid boilerplate configuration.
(even if it should still be possible to define multiple sources with same account/file, and different folders, depending on the need).
There should also be an option, to allow including //all// existing folders (and subfolders) for reading. Would be better to define this as an mutually exclusive options (either you specify a folder or a list of folders, either you include them all)

The archive should be able to open the folders in turn for the same source, find a specific email from a source from any of its folders, etc...