Use system-specific Palaso assemblies

The Palaso libraries have a fair amount of '#if MONO' conditional
compilation in the source code, thus requiring separate Windows and
Linux versions of the assemblies.  This fixes FWNX-1054 (and FWNX-1051
as well since the Linux Lift.Media.dll has fixes for handlng sound files
better).

Change-Id: I36bad7db5f8023b5bda59c449df323a5c17df50f