smarter display of units.

Currently units display is set in the scene. However some numbers can get very long and become hard to read and manipulate. Some people work in scene of 2 km size and still need to adjust millimeters. So it would be cool if the display of numbers would act more smart. So instead of 0.001 m, it would be automatically displayed as 1 mm. That woulds also make input lot easier, if you want to move it to 3 mm, just change the 1 in a 3 before the 'mm'