Real-world units of measure

[alezzacreative]

i would love us to have this very quick way to change the units in the ruler (its common in adobe products) an inkscape dev added it just yesterday after alot of user requests.
https://user-images.githubusercontent.com/63913433/163684811-305787d7-88ce-4dc8-9410-42c14d19baa9.mp4
thank you

[Keavon]

This should be one of the document settings, which we'll need to make a menu for, to set the relationship between resolution and real-world units, as well as the default units to display. We should have a math-in-the-inputs-boxes system which does automatic unit conversions as well, so you can type any unit and it converts to dimensions then displays in the default display units.

[Keavon]

We should also have a way to let node parameters take a choice of multiple types of units. For example, the Resample Points node has Offset Start and Offset Stop parameters that currently accept a distance value. It would be good if we could let that node accept either a distance or a percentage, where the percentage is what factor along the curve's length to use as the start/stop offset. Percentage units should also work based on either a factor (0-1) or percent (0%-100%) as equivalent ways to express the same unit.