Fixes for mass_tagger extension

Fixed a bug where the mass_tagger extension would behave incorrectly if the id of an image was the same as the ending of the id of another image. This caused the wrong images to be tagged.

Example:
Select image 143: var string == "143:"
Select image 113: var string == "143:113:"
Select image 43: var string == "1113:" (instead of "143:113:43:")

Also, now the extension recognizes negative tags and removes them from the selected images.