Replacement for commits bc47054b and ad0d0f6d

Replacement for commits bc47054b and ad0d0f6d

The strategy adopted in bc47054b had some drawbacks related to the way
instant preview snippets are generated. See the subthread starting at
http://www.mail-archive.com/lyx-devel@lists.lyx.org/msg187916.html
for details.

The strategy adopted in this commit is that of adding macro definitions
only for the macros actually used in a preview snippet, independently
of whether some macro was already used in a previous snippet. In this way
the snippets don't need to be changed according to whether they are
compiled as a whole or separately from each other. This fact was causing
the regeneration of a preview snippet whenever the cursor entered the
corresponding inset, even if the generated image would have not changed.
The problem of defining or redefining a macro is taken care by the
python scripts.