Bugfix in ListMailboxes call

[FStaal]

As TIdIMAP4.ParseIntoBrackettedQuotedAndUnquotedParts is being used in other parts of teh code I opted for changing TIdIMAP4.InternalParseListResult. As discussed here: https://stackoverflow.com/questions/65022293/bug-in-tidimap4-listmailboxes/65029645?noredirect=1#comment115098806_65029645 I've changed it to use the index 2 as the FMailBoxSeparator and everything following that as the MailboxName.