object: Add support for signal forwarding

The CAPI signal system already has this mechanism.

lib/gears/object.lua

@@ -56,6 +56,11 @@ function object:connect_signal(name, func)
     sig.strong[func] = true
 end
 
+-- Register a global signal receiver.
+function object:_connect_everything(callback)
+    table.insert(self._global_receivers, callback)
+end
+
 local function make_the_gc_obey(func)
     if _VERSION <= "Lua 5.1" then
         -- Lua 5.1 only has the behaviour we want if a userdata is used as the
@@ -120,6 +125,9 @@ function object:emit_signal(name, ...)
     for func in pairs(sig.weak) do
         func(self, ...)
     end
+    for _, func in ipairs(self._global_receivers) do
+        func(name, self, ...)
+    end
 end
 
 local function get_miss(self, key)
@@ -188,6 +196,8 @@ local function new(args)
 
     ret._signals = {}
 
+    ret._global_receivers = {}
+
     local mt = {}
 
     -- Look for methods in another table