There are many format variables now so allocating all the default ones

There are many format variables now so allocating all the default ones
each time a tree is created is too expensive. Instead, convert them all
into callbacks and put them in a static table so they only allocate on
demand. The tree remains for the moment for extra (non-default)
variables added by for example copy mode or popups. Also reduce
expensive calls to localtime_r/strftime. GitHub issue 2253.