Emit property::screen in tag:delete()

Before this commit, the code directly modified the table where the tag's
properties were saved. This commit changes the code to call
awful.tag.setproperty() instead. This function ensures that
property::screen is now also emitted.

Signed-off-by: Uli Schlachter <psychon@znc.in>

lib/awful/tag.lua

@@ -402,7 +402,7 @@ function tag.object.delete(self, fallback_tag, force)
     end
 
     -- delete the tag
-    self._private.awful_tag_properties.screen = nil
+    tag.setproperty(self, "screen", nil)
     self.activated = false
 
     -- Update all indexes