Move folder directories in own "folders" subdirectory, rename spam folder to "Junk"

Here are some thoughts about our current folder management and how we can make it more compatible and logical:

1. despite cluttering the main YAM directory (YAM with all mail folder directories, it would be better to have a global YAM:folders directory in which new and existing folders are moved/created in.
2. The default folders (outgoing/incoming/sent/spam, etc.) should be renamed (the directories) to be compatible to what other mail clients (e.g. Thunderbird) name them. This would make it more easy to integrate IMAP support in future. This would me to rename "outgoing" to "Outgoing" or "sent" to "Sent". And also rename "Spam" to "Junk".

This would of course also require to implement an automatic folder move operation once 2.7 is started and found old default folders lying in the YAM: directory instead of having them in the YAM:folders subdirectory as well as automatically renaming e.g. "Spam" to "Junk" and so on.

After all, this change would really make the folder management ore consistent for future changes. Currently, the YAM: directory is really cluttered with many mail folder directories which is not nice IMHO.