unfold next level for current tree
fold one level for current tree
handle nil next-tree-line so fold/unfold current tree works at EOF
organize editor-based helpers

Also refactor find-first to take explicit editor
objects to avoid refetching latest editor.
reuse safe-next-non-child-line

Also put exact :ch for current-tree
clean up fold-all/unfold-all to reuse editor object
clean up distinction between levels and indents

Was getting sloppy
rename fn to be more understandable