Custom grid divisions and intuitive lengths when working with imperial scene units

When working in imperial units, it's helpful to know where the inch relative to the foot relative to the yard are, as well as various sub-inch increments depending on your project needs. As such, the user should be able customize grid divisions when scene units are set to imperial.

This functionality could be implemented in one of two ways:

1. When the scene units are set to imperial, the shader menu would provide the user with a plus icon under the grid division field which would allow the user to implement custom divisions on a per order of magnitude basis.

   By default, the initial division field would be set to 1 and accompanied by a quantifier set to 3 (measured in feet). This default setting would result in divisions occurring once per yard, since it specifies one division per three feet. Zooming in would not reveal finer divisions, however the user would be able to add a division field & quantifier combo below the first in the shader menu using the plus icon. Here is how it would work:

   If they set the field to 3 and leave the quantifier alone, the quantifier would default to whatever length is represented by the field/quantifier combo above it (in this instance, one yard), and three divisions would occur along that length. As the user zooms in, the yard divisions would become brighter, appearing as major divisions, and new divisions would appear, having ordinary brightness and occurring once per foot.

   Alternatively, the quantifier could be set manually for one, more than one, or each new field. For instance, if the user wanted their yard divided up into inches instead of feet, they could set the new field to 12 instead of 3 and the quantifier (measured in feet) to 1.

   More likely than not though, the user will want their yard divided into feet and their feet divided into inches. So, discarding the alternative scenario mentioned above, the user would be able to achieve this by using the plus icon again to add another new field/quantifier combo, setting the new field to 12 and leaving the quantifier alone. Upon zooming in the foot divisions would transition into major divisions while inch divisions would fade gradually into visibility.

   At this stage, if the process were repeated one more time, the user could hot swap between sub-inch divisions such as 1/2, 1/4, 1/8 and so on based on the momentary needs of their project by setting the field to 2, 4, 8 etc. and leaving the quantifier unset.

2. Or alternatively, when the scene units are set to imperial, the custom grid divisions used in the above example would be the default behavior of the grid shader. The shader menu would not provide the user an option to customize grid divisions at every order of magnitude. Instead, the user would be presented with set radios for sub-inch divisions only, such as 1/2, 1/4, 1/8, 1/16 1/32, 1/64 and 1/128.

Finally, when the scene units are set to imperial, the scene > units menu should provide users with (in addition to the separate units option that already exists) an "Imperial CAD" mode. I don't know what it would be called. But it would make the UI handle distance and measurement readouts in a way that woodworkers are familiar with; for example, 3 feet, 8 inches and fifteen 16ths of another would appear as 3' 8" 15/16", but only when the length being represented is precisely that -- If a length is nearly 31/32" but not exactly, and cannot be represented by 64ths or 16ths, the UI should default back to representing it as a decimal.