ROI/measurement tool fixes

Fixes some issues with the ROI/measurement tool:
* 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/  (this has already been reviewed by @aleksandra-tarkowska in #3070 , probably no need to review again)
* Wrong measurements with images with non-micron pixel sizes. Measurements were always calculated in micron size, even if the reference unit was nm. To test: Open an image with nm pixel size, draw a line and check that the distance measurement is correct, also check that unit and value is correct on the results tab. See https://www.openmicroscopy.org/qa2/qa/feedback/9560/
* Only PNG export was available via the context menu on the LineProfile chart (now PNG and JPEG is available); reported by @pwalczysko, see Trello card https://trello.com/c/h0PLjn5r/54-more-roi-fixes

I created a bit of a mess with my different ROI fix branches/PRs. So I moved all relevant commits into this PR and going to close #3070.