ThresholdAdjuster Improvements

Thanks to Michael Schmid, the "Threshold" dialog in ImageJ 1.48r has the following improvements and bug fixes:

• Shows the percentile of thresholded pixels! (based on the histogram; thus an approximation for 16-bit & float images). In over/under mode, it shows the over & under percentages.
• Bringing the "Threshold" widget to the front updates the histogram and threshold (if any) to the values of the current foreground image.
• If the image or the roi is changed while the "Threshold" widget is in the foreground (via keyboard shortcuts), the histogram reflects the changes after the next operation of the "Threshold" widget (clicking a slider without moving it is enough).
• If there is no threshold, none is shown in the histogram.
• Changing the 'Black Background' settings in Process>OptionsBinary>Options does not autothreshold the currently active image (it does ThresholdAdjuster.update()
• The histogram display shows all 256, not only 255 values (the leftmost value was hidden behind the black rectangle). Also, bins with very few counts (>=1) still get a one-pixel-high line.
• The histogram uses the color LUT or inverted LUT not only for 8-bit but also for 16 bit and float images.
• The histogram is still visible where the LUT color is almost white or white.
• Undo is no longer disabled.
• On Macs, if the "Threshold" widget is the front window the menus are less likely to disappear.