kcms/nightcolor: Fix DayNightView on edge cases

Improve the DayNightView model for some edge cases where the
transition ends on the next day than it starts

- Hide the edge icon when a transition overflows
- Calculate better if the middle icon represents day or night
- Move the gradient stop for position 0 to the end of the list,
  to avoid other stops overriding it at the same position.

BUG: 482032
FIXED-IN: 6.0.1


(cherry picked from commit f17f0543)