CellWorx: ignore thumbnails and missing channel names

Any TIFF files with "_thumb_" in the name are now ignored, so that
thumbnails are not inadvertently picked up.  If channel names are
missing from the .htd file, then the channel names stored in the
.pnl/.tif files will be used instead.
Merge pull request #746 from melissalinkert/metamorph-screens

CellWorx: ignore thumbnails and missing channel names