ForecastSidebar: allow toggling between today and tomorrow

Remove the heuristics for choosing today or tomorrow in the sidebar,
and always show today if possible. Then allow switching to the next
day and back with the button at the end.
This should solve the ambigous cases close to the end of the day,
when the heuristics would not kick in but little or no useful
information was being shown.
ForecastsBox: improve filtering and grouping

Instead of filtering dates that are separated by at least 12 hours,
separate two dates if they would give different "Day Part" strings.
Also, slightly tweak the mapping between hours and day parts,
and filter out the night part before early morning. This way, we
make sure the night never comes before morning (which makes sense
in some locales, but not in others, and maybe confusing in general)