Expose the 'Slide Show' plugin from the Macro Interpreter

The Macro Interpreter adds a command 'show' which does not work
otherwise.

Noticed by Jan Eglinger in bug 474.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Merge remote-tracking branch 'github/bug474'

Tested by Jan Eglinger. See http://fiji.sc/bugzilla/show_bug.cgi?id=474

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>