Work with date ranges instead of individual years

[eichblatt]

The 78 rpm program currently works with an individual year. Extend this to work with a range of years

[eichblatt]

Pressing the Year knob should set the currently staged year as the year_A, and then turning the Year knob will begin staging year_B.
When the select or Year button is pressed, the date range will be from
start_year to end_year where
start_year = min(year_A, year_B) and end_year = max(year_A, year_B)

The range will be displayed as SSSS - ee
where SSSS is the 4-digit start_year and ee is the last 2 digits of the end_year

if start_year == end_year then the range will be displayed as simply SSSS