Make argument of slots explicit in examples

Make the name of arguments explicit by using the

onXXX: function(arg) {

syntax.

Change-Id: I1b24e444d4c8e2e26561edc043b53672ccb4de23
Reviewed-by: Leena Miettinen <riitta-leena.miettinen@qt.io>
Reviewed-by: Michal Klocek <michal.klocek@qt.io>