Previously showMixedAmountElided would show two amounts and then the
elision string if necessary. Now it will display as many Amounts as it
can subject to the condition that the amounts plus the elision string
fit within 22 characters.
Multicolumn balance reports showing many commodities tend to become
unreadably wide, especially in tree mode. Now by default we show at
most two commodities, and a count of the rest if there are more than
two. This should help keep reports somewhat readable by default.