Fix bug ##9627: Make the document name available to preview insets

Fix bug ##9627: Make the document name available to preview insets

The \jobname macro is redefined for the preview snippets such that
it contains the name of the document without extension. This is
harmless because the moment it is redefined, TeX has already chosen
the name of the output and log files. On the other hand, any use
of \jobname in preview snippets is now consistent with what one
obtains after exporting and compiling by hand.