Adds thumbnail generation for WebP images

This resolves issue #380

Signed-off-by: Alex Kent Hajnal <software@alephnull.net>

tdeioslave/thumbnail/imagethumbnail.desktop

@@ -4,6 +4,6 @@ Type=Service
 Name=Images
 
 X-TDE-ServiceTypes=ThumbCreator
-MimeTypes=image/cgm,image/fax-g3,image/gif,image/jp2,image/jpeg,image/pjpeg,image/png,image/tiff,image/x-bmp,image/x-dds,image/x-ico,image/x-jng,image/x-pcx,image/x-photo-cd,image/x-portable-bitmap,image/x-portable-greymap,image/x-portable-pixmap,image/x-rgb,image/x-targa,image/x-wmf,image/x-xbm,image/x-xcf-gimp,image/x-xfig,image/x-xpm
+MimeTypes=image/cgm,image/fax-g3,image/gif,image/jp2,image/jpeg,image/pjpeg,image/png,image/tiff,image/webp,image/x-bmp,image/x-dds,image/x-ico,image/x-jng,image/x-pcx,image/x-photo-cd,image/x-portable-bitmap,image/x-portable-greymap,image/x-portable-pixmap,image/x-rgb,image/x-targa,image/x-wmf,image/x-xbm,image/x-xcf-gimp,image/x-xfig,image/x-xpm
 X-TDE-Library=imagethumbnail
 CacheThumbnail=true