[Feature request] different units

[gdkrmr]

Here are some useful units:

• Percent, ruler should be 100% length. Maybe a way to freeze percent and to extend the ruler beyond 100%
• Inch/cm. Requires reading out monitor dpi. Very useful when using LaTeX and having to position stuff on a pdf.

[epilys]

There was some relevant work in progress done in the ppi branch, maybe we can restart it :)