Annotation view Ruler Hand-tool

[sbatchelder]

If a media type has a 'pixel-resolution ratio' as defined/discussed in issue #1836, a new hand-tool should be made available in the annotation view: the ruler. This tool would allow a user click two or more points on the image (full or zoomed in), creating line segment(s), the converted real-length value of which gets calculated and displayed. Clicking off the tool dismisses the ruler line segment and reported measurement.