tasklist: Add a `count` property.

This is mostly for the signal handler. It will allow to do things
like replace the template or layout based on the number of clients.

lib/awful/widget/tasklist.lua

@@ -491,6 +491,11 @@ local function tasklist_update(s, self, buttons, filter, data, style, update_fun
         end
     end
 
+    if self._private.last_count ~= #clients then
+        self:emit_signal("property::count", #clients, self._private.last_count)
+        self._private.last_count = #clients
+    end
+
     local function label(c, tb) return tasklist_label(c, style, tb) end
 
     update_function(self._private.base_layout, buttons, label, data, clients, {
@@ -499,6 +504,13 @@ local function tasklist_update(s, self, buttons, filter, data, style, update_fun
     })
 end
 
+--- The current number of clients.
+--
+-- @property count
+-- @readonly
+-- @tparam number The number of client.
+-- @propemits true false
+
 --- Set the tasklist layout.
 --
 -- @property base_layout
@@ -559,6 +571,14 @@ function tasklist:set_base_layout(layout)
     self:emit_signal("property::base_layout", layout)
 end
 
+function tasklist:get_count()
+    if not self._private.last_count then
+        self._do_tasklist_update_now()
+    end
+
+    return self._private.last_count
+end
+
 function tasklist:layout(_, width, height)
     if self._private.base_layout then
         return { base.place_widget_at(self._private.base_layout, 0, 0, width, height) }