properties: Allow to delay some operations.

This will be useful for the append/remove support.

lib/gears/object/properties.lua

@@ -8,6 +8,7 @@
 ---------------------------------------------------------------------------
 
 local gtable = require("gears.table")
+local gtimer = nil --require("gears.timer")
 -- local gdebug = require("gears.debug")
 local object = {}
 local unpack = unpack or table.unpack -- luacheck: globals unpack (compatibility with Lua 5.1)
@@ -83,6 +84,8 @@ function object.capi_index_fallback(class, args)
         end
     end
 
+    assert(type(class) ~= "function")
+
     -- Attach the accessor methods
     class.set_index_miss_handler(getter)
     class.set_newindex_miss_handler(setter)
@@ -190,7 +193,7 @@ local function copy_object(obj, to_set, name, capi_name, is_object, join_if, set
     })
 end
 
-function object._legacy_accessors(obj, name, capi_name, is_object, join_if, set_empty)
+function object._legacy_accessors(obj, name, capi_name, is_object, join_if, set_empty, delay)
 
     -- Some objects have a special "object" property to add more properties, but
     -- not all.
@@ -248,6 +251,7 @@ function object._legacy_accessors(obj, name, capi_name, is_object, join_if, set_
 
 
         --TODO v6 Use the original directly and drop this legacy copy
+        local function apply()
             local result = is_formatted and objs
                 or gtable.join(unpack(objs))
 
@@ -258,6 +262,23 @@ function object._legacy_accessors(obj, name, capi_name, is_object, join_if, set_
             else
                 self._private[name.."_formatted"] = result
             end
+        end
+
+        -- Some properties, like keys, are expensive to set, schedule them
+        -- instead.
+        if not delay then
+            apply()
+        else
+            if not self._private["_delayed_"..name] then
+                gtimer = gtimer or require("gears.timer")
+                gtimer.delayed_call(function()
+                    self._private["_delayed_"..name]()
+                    self._private["_delayed_"..name] = nil
+                end)
+            end
+
+            self._private["_delayed_"..name] = apply
+        end
 
         self._private[name] = copy_object(
             obj, objs, name, capi_name, is_object, join_if, set_empty