Missing features:
- populate input area (still needed: mm/mil, rezoom)
- add table/var/loop editor (missing: add col/row, add/del var/table/loop)
- add default unit (combine with grid unit selection ?)
- consider adding auto/mm/mil selection for each dimension
- add measurements (partly done. still needed: find out how to define
  non-trivial endpoints, e.g., some vector in last iteration of loop)
- add KiCad output
- add postscript output
- add option to include/omit helper vecs and frames (display and postscript)
- define part name

Error detection:
- eliminate duplicate instances

Style:
- make column of entry field greedily consume all unallocated space
- status area looks awful
- status area bounces when something becomes editable

Bugs:
- default silk width has no business being hard-coded in obj.c
- after moving, arc sometimes wrap the wrong way
- undelete only works if not much has changed since the deletion

Code cleanup:
- merge edit_unique with edit_name
- merge find_var_in_frame with similar mechanisms in expr.c and fpd.y
- add regression tests

Open decisions:
- Q: should loop be (start, last) or (start, iterations) ? or start ... last ?
- change vector circle color ? (also, highlight on hover ?)
- Q: allow reassignment of vector names ?
  A1: no: would cause confusion in GUI (vectors could become orphaned)
  A2: yes. but we don't change the linkage.
- Q: how do we handle stacks of objects ?
  A: we don't but we make it easy to avoid them, by giving a good zoom,
     flexible selection, and by disallowing stacks of identical objects in the
     first place.
- Q: add frame arguments ? (e.g., .frame pad(pin_num_offset) ...)
  we can already approximate this by introducing an intermediate table that
  sets up the arguments (provided that we don't consider vectors as well)
- Q: should we make it a requirement to generate objects only once ?
  A: almost certainly yes.

Future directions:
- future: consider using cairo instead of gdk
- live update of value when entering strings and expressions ?
- advanced: non-standard solder mask
- advanced: solder paste exceptions (subtractive, additive)
- advanced: silk line width
- future: when encountering an error after a change, we could try to find the
  same element in the old instance, and select it
- future: consider editing off-canvas items in place