Make path separators for sidebar folders configurable.                                                                                                                                                                            
                                                                                                                                                                                                                                  
When using IMAP, a '.' is often used as path separator, hence make the                                                                                                                                                            
path separators configurable through sidebar_delim_chars variable.                                                                                                                                                                
It defaults to "/." to work for both mboxes as well as IMAP folders.  It                                                                                                                                                          
can be set to only "/" or "." or whichever character desired as needed.


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When using IMAP, a '.' is often used as a separator instead of '/'.
This patch enables mutt to find these dots and
1. correctly intend the dir in the sidebar
2. if "sidebar_shortpath" is set, shorten the dir to the part after
   the last dot