IMAP: New folders will now adopt default mail attributes.

* Added a function CopyMailFolderAttributes() that copies the attribute
layout from the text/x-email default query folder.
* This using the new CopyAttributes() method in libshared that is pretty
much a copy of a similar method from copyattr. However, I did not
replace the latter, as that one allows for more fine grained error
reporting (and attribute filtering).
* Closes ticket #3498.