Scalebar

This PR allows to display scalebar in the viewport. It is supported for both regular and big images.

To be tested with regular and big images. Cross browsers

To test it open images where physical pixel sizes are in various units (nano, micro, etc), click checkbox and see if  scalebar appear with appropriate length and value round to power of 10. Then zoom in and out and check if scalebar update accordingly. Scalebar should be updated dynamically. Length and the value will update to the nearest suitable that is round to power of 10.
What is ```scalebar_displayed``` used for?