widget: Add class-level signal forwarding.

Just like clients and other CAPI classes, it is now possible to
connect to all instance signals. There was already a couple of
`request::geometry`, but no way to forward them, so it was
de-facto broken.

lib/wibox/init.lua

@@ -388,6 +388,9 @@ local function new(args)
         ret.input_passthrough = args.input_passthrough
     end
 
+    -- Make sure all signals bubble up
+    ret:_connect_everything(wibox.emit_signal)
+
     return ret
 end
 
@@ -409,6 +412,8 @@ object.properties(capi.drawin, {
 
 capi.drawin.object = wibox.object
 
+object._setup_class_signals(wibox)
+
 return setmetatable(wibox, wibox.mt)
 
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