Resolves title/subtitle/caption/tag labels for plot from (in
increasing precedence):
the xpose.xtras.default_labs R option (a named list, see examples),
defaults set on xpdb via set_default_labs(), when xpdb is
supplied (a rendered plot does not retain enough of its source
xpdb to look this up automatically -- pass it explicitly),
values passed directly via ....
By default only labels not already set on the plot are filled in; set
overwrite = TRUE to replace existing labels too.
print.xpose_plot() calls this automatically
whenever xpose.xtras.auto_apply is enabled (the default; see
set_xtras_options()), but apply_default_labs() itself is a
standalone function that doesn't depend on that print method existing,
so it works the same regardless of whether that method survives issue
#36's eventual removal.