Allow all thumbnails generation size

It would be usefull to allow users to generate all size thumbnails.

A simple way would be to add a new parameter " thumbnail.allow_all_size=true" in WEB-INF/etc/slide/listener.properties and then change the test in class
org.jahia.services.webdav.listeners.IMGSlideContentListener to create the thumbnail and to not return if allow_all_size is true.

if (!props.getProperty("thumbnail.size").equals(s))

{ return; }

So the image /jahia/webdav/site/site1/shared/image.jpg could be resize directly to 200px by calling it with /jahia/webdav/site/site1/shared/image_th_200.jpg