[Users] Customize path for impacache

[Redfox1000 at gmx.de]

Hei Brian,

> I don't know where else you would expect them to be.

guess I have a distorted understanding of how to deal with IMAP. For
me the synchronized messages aren't really "cache". They are the data I
almost exclusively deal with on this machine. And therefore I want to
"see" them in my file manager. Furthermore config files are pretty
small and should be put in hidden directories but not so with mails.
To me, it just doesn't feel right that .claws-mail contains several GB.
Yes, claws should deal with big amounts of data. But this data should
be stored "more accessible".

Maybe it's naive to want some kind of combination of IMAP and POP3. But
for my needs it would be sufficient just to change the path of the
cache folder.

Matthias