ROI tool fixes

Fix for two ROI issues:
* Multi-Z and/or multi-T images: If the ROI tool was opened once, moving the Z and/or T slider continuously reopenend the ROI dialog, even it has been closed before. See https://www.openmicroscopy.org/qa2/qa/feedback/9527/
* Images with nm pixel sizes: Length values were shown in microns, rounded to two decimal places, which made length measurements for these images pretty useless. With this PR micron values < 1.0 will be shown as nm.  See https://www.openmicroscopy.org/qa2/qa/feedback/9560/