Fixed input_passthrough property not being set (#2723)

* Fixed input_passthrough property not being set

In the table of properties supplied to the `wibox` function, you couldn't set the `input_passthrough` property. You could only set it after the wibox was created like this: `my_shlick_wibox.input_passthrough = true`. This commit fixes that and now you can set it in both ways.

lib/wibox/init.lua

@@ -340,7 +340,7 @@ local function new(args)
         ret.shape = args.shape
     end
 
-    if args.screen then
+    if args.input_passthrough then
         ret.input_passthrough = args.input_passthrough
     end