Add a format and type argument to the set_prop functions

This will allow the addition of command-line options to set format and
type, and the reuse of this code for the set-{atom,float,int}-prop
paths.

Signed-off-by: Julien Cristau <jcristau@debian.org>
Signed-off-by: Peter Hutterer <peter.hutterer@who-t.net>