Correct converter configuration for inkscape on Mac

Correct converter configuration for inkscape on Mac

On Mac the inkscape binary is started by a wrapper script. This script changes the
working directory internally and fails to process files with relative path names.
The previous attempt to solve it was to pass the file names with absolute names
by prepending them with the $$p variable (representing the directory name of the files).
This broke the on screen conversion (used for SVGZ to PNG e.g. in the users guide)
because here the $$p variable is undefined.

Now the wrapper script of LyX which is used to locate the Inkscape.app bundle converts
the relative path names into absolute names and the $$p variable is removed from the
converter definitions for inkscape again.

(cherry picked from commit caa1dd2aeed97330e05c0e96ae7e5bb929b0866d)