[Thresholding] Toggling between 'on-screen images' and 'image directory' should reflect in slider/thresholding method updates

**Please describe the problem that needs to be solved by this feature**
Currently when image directory is selected, if the thresholding slider is moved or a new thresholding method is selected, the viewer updates with the new thresholds. Might be confusing to user. Should disable and only enable thresholding updates to viewer when on screen images are selected.