added subfolder support

[ajkessel]

Protonmail provides a label metadata field path which includes the full path of the folder. This commit adds support for path and uses path if it exists, thus preventing the situation where folder x/z appears to be the same as folder y/z. It then converts the slashes in the paths to dots when presented to the IMAP client. A better implementation would use the IMAP RFC to show parent and child folders and shouldn't be too hard to do, but this at least allows the user to see their fully qualified folders in their IMAP client for the time being, rather than having an erroneous presentation.