Always periodically dispatch pending items

"Local" URLs can also be slow.
This might jeopardize loading large local directories where a resorting could take place but imho it's better than leaving the user
wondering when files will finally show up. However, it takes some time for KDirLister to initially signal items having been added,
and only then our 2 second timer starts.
Also, stop the timer when loading has completed.

Differential Revision: https://phabricator.kde.org/D23460