One problem is your specified "icons" property doesn't match the name of your toolbar button.
From this tutorial, the relevant quote is:
Another problem is that in the toolbar config, you need to put the name of the button, not the name of the plugin.
From checking an official plugin ("Source Dialog"), I see they went with "Sourcedialog" for the button name (and thus "sourcedialog" for the icons property). So you could change your button name from "Loop Video" to "Loopvideo" and that would fix it (since your icons property would now be the lowercase version of that). Don't forget to also change the button name in the toolbar group config just in case, since I'm not sure if it's case-sensitive.
Also, after making these changes, remember that your browser will still probably have the old version cached.
