[BUG] LaTeX preview should use a subdirectory in /tmp

[Max Nikulin]

This is a follow-up to recent changes related to LaTeX preview.

This feature should not write temporary files to /tmp directly, some 
subdirectory should be created for this purpose. The idea is to mitigate 
consequences if a user opens a file from a compromised or a malicious 
source and gets /tmp flooded with a crowd of files. It is easier to 
delete single directory than to spent time trying to figure out what 
files are necessary for other applications and what ones are generated 
by LaTeX code.

P.S. I do not mind double level structure: all temporary Org mode files 
(babel, etc.) are created in single (per Emacs process) directory in tmp 
and each task creates its own subdirectory there.