awful.tag: Add "volatile" property

Useful when using dynamic tags. The tags will be closed once
it is empty. This was part of Tyrannical for many years, but is
generally useful for other workflows too.

   local t = awful.tag.add("my_tag",{volatile=true, screen=2})
   awful.spawn("ayapp", {tag=t})

lib/awful/tag.lua

@@ -437,6 +437,20 @@ function tag.setlayout(layout, t)
     return layout
 end
 
+--- Set if the tag must be deleted when the last client is untagged
+-- @tparam boolean volatile If the tag must be deleted when the last client is untagged
+-- @param t The tag to modify, if null tag.selected() is used.
+function tag.setvolatile(volatile, t)
+    tag.setproperty(t, "volatile", volatile, true)
+end
+
+--- Get if the tag must be deleted when the last client closes
+-- @param t The tag to modify, if null tag.selected() is used.
+-- @treturn boolean If the tag will be deleted when the last client is untagged
+function tag.getvolatile(t)
+    return tag.getproperty(t, "volatile") or false
+end
+
 --- Set the spacing between clients
 -- @param useless_gap The spacing between clients
 -- @param t The tag to modify, if null tag.selected() is used.
@@ -817,6 +831,10 @@ local function client_untagged(c, t)
     if c.urgent then
         update_urgent(t, -1)
     end
+
+    if #t:clients() == 0 and tag.getproperty(t, "volatile") then
+        tag.delete(t)
+    end
 end
 
 -- Count the urgent clients.
@@ -846,6 +864,7 @@ capi.tag.add_signal("property::screen")
 capi.tag.add_signal("property::index")
 capi.tag.add_signal("property::urgent")
 capi.tag.add_signal("property::urgent_count")
+capi.tag.add_signal("property::volatile")
 
 capi.screen.add_signal("tag::history::update")
 capi.screen.connect_signal("tag::history::update", tag.history.update)