awful.widget: Add an helper to set a property on all template widgets

With this helper, it becomes possible to avoid manually setting common
properties such as the client in the tasklist of tag in the taglist when
the children widgets of the template have a set_+property_name.

lib/awful/widget/common.lua

@@ -91,6 +91,19 @@ local function default_template()
     }
 end
 
+-- Find all the childrens (without the hierarchy) and set a property.
+function common._set_common_property(widget, property, value)
+    if widget["set_"..property] then
+        widget["set_"..property](widget, value)
+    end
+
+    if widget.get_children then
+        for _, w in ipairs(widget:get_children()) do
+            common._set_common_property(w, property, value)
+        end
+    end
+end
+
 --- Common update method.
 -- @param w The widget.
 -- @tab buttons