Simple thumbnails browser for 5.x relying on imagecache

This patch add a thumbnail on the last column of the image browser on 5.x-1.2 version of this module using ImageCache module where this feature lacks.

A new text field appears on admin/settings/imce and you have to fill it with your Imagecache preset name to show the thumbnails (you can change it anytime). It doesn't affect inserted images, but it provides only a thumbnail to facilitate image choosing.