CellWorx: ignore thumbnails and missing channel names (rebased onto develop)



This is the same as gh-746 but rebased onto develop.

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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.

/cc @chris-allan