FileinputCssReference - img/loading.gif cannot be located

[bkempe]

A ResourceReference wont be created for a resource with key [scope: de.agilecoders.wicket.extensions.markup.html.bootstrap.form.fileinput.FileinputCssReference; name: img/loading.gif; locale: null; style: null; variation: null] because it cannot be located.

This looks like a regression of #554.
The img urls moved to img/ in the Bootstrap FileInput update to 4.3.5 but the files weren't moved back.

[martin-g]

Please create a Pull Request with the needed changes!
Thank you!