Allow protocol-specialized ThumbCreators to be used for folder previews

Review Request #103882 - Created Feb. 7, 2012 and submitted - Latest diff uploaded

Sebastian Trueg
The problem is fairly simple: I needed to create previews for directory items in a custom KIO slave. Writing the ThumbCreator was simple. But it got never called. The reason is that the thumbnail kio slave always uses its own directory thumbnailing method for folders if the plugin name is not specified in the metadata. The PreviewJob on the other hand does only specify the "plugin" metadata field for "local" ThumbCreators; remote ThumbCreators are only checked for existance.
This patch fixes that by first checking for a specialized ThumbCreator plugin by protocol and then falling back to the old way of detection my mime-type.
Tested on my custom KIO slave: perfect thumbnails. Thumbnails for everything else still working fine.