Symbol loaded as a sheet
~~~~~~~~~~~~~~~~~~~~~~~~

Also known as: symbol edit mode or symedit.

When this happens, sheet->is_symbol is set to 1 which triggers a bunch of
special cases here and there. The general structure is this:

- a symbol file is a single group (stored under a csch group in lihata)
- when the loader detects this root, this group is loaded as sheet->direct
- in sch-rnd sheet postproc some extra steps are done to load pens from the
  default sheet, else all pens would be invalid; these pens are marked
  ->copied_from_default=1
- on save sheet->is_symbol decides only the direct group needs to be saved,
  using the csch group root node instead of a sheet root
- on save pens that are marked ->copied_from_default==1 are not saved
  (undoable pen modify always resets this bit so any pen modification forces
  saving the pen)