Add \save_transient_properties parameter (#9841)

Add \save_transient_properties parameter (#9841)

Increment LyX format to 504.

With this new parameter, the user can indicate that some other parameters that
are frequently switched must not be recorded in the file (as if they were a
setting specific to the user or transient, rather than a document setting). This
will play nicer with version control systems.

See the discussion, e.g.:
http://thread.gmane.org/gmane.editors.lyx.devel/157824/focus=157993 (third
solution mentioned)

TODO:
* The interface remains to be set up. We cannot change this setting from LyX
  for now.
* If save_transient_properties is false, we should read the user setting as a
  per-user-per-document (session) setting (e.g. like the cursor position).
* Once the above is done, we can treat \justification the same way (but it would
  be even better if \justification was moved to lyxrc).