Provide a way to "zero" the meter on a measurement target

[dkonigsberg]

It is a common use case to want to measure density relative to some "base" (unexposed film/paper area), rather than relative to the ISO-standard "zero" point. Having this ability would make it a lot easier to collect and compare measurements in the most common use cases.

As such, the this capability should be implemented as follows:

• Have a way to set the current reflection/transmission reading as the "zero" offset for that mode, from either the display or measurement states
• Make sure the "zero" offset is simply a linear offset applied to the result, and not used as part of the actual logarithmic density calculations
• Indicate (in the display state) whether a "zero" offset has been set
• Have a way to clear a currently-set "zero" offset, most likely from the display state.
• Make sure the "zero" offset is specific to the current measurement mode, and is not persisted across device restart. It is not considered part of device calibration.
• Implement display of negative density readings, to handle cases where a measurement is below the "zero" offset point.

[dkonigsberg]

Initial implementation provided by commit: f6bdefde03eea467675780d40794cb0f69a6a169

Probably needs more polish, which might depend on fancier key input handling first.