Allow to drag, resize and position scale bar

[jmrk84]

This would be a very useful feature for simple measurements in the skeleton and data viewports. A "reset scale bar" button would then be required of course. I imagine to click one end of the scale bar, hold the mouse clicked and then move this end to a different position, elongating or shortening the scale bar with this operation.

[orenshatz]

Maybe we should also have an node-independent arbitrary two-point/cumulative distance measurement. It can easily be implemented with a plugin.

[orenshatz]

Regardlessly - I wonder about diagonal (arbitrary angle) scale bar. In principle this would be ideal for dynamic-bar measurements, but in the Z viewports the anisotropy is a problem. What do you think?