Remove "Show only days in current month" month view option

This was intended to be hooked up to Preferences but with our
current design it doesn't seem likely, in particular the way
California displays all-day and day-spanning events in the month
view.
Easier conversion of Calendar.ExactTime to Calendar.WallTime

Old way used a name constructor in WallTime.  Since these conversions
are often chained with adjustments, a ExactTime.to_wall_time() is more
natural.
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