- added support for string constants to unparse()

- implemented eval_str
- expand() now tries to obtain a string
- added example fbga.fpd to demonstrate use of strings
- when invoked with an inexisting file, fped now starts with an empty model,
  instead of getting confused 
- we now religiously call edit_nothing before adding fields to edit, so that
  we won't create a loop through edit-next



git-svn-id: http://svn.openmoko.org/trunk/eda/fped@5421 99fdad57-331a-0410-800a-d7fa5415bdb3

TODO

@@ -39,6 +39,9 @@ Code cleanup:
 - code organization is very poor. E.g., functions belonging to the different
   items (pads, silk objects, vectors, etc.) should be grouped by item, not by
   type of function, similar to how some things are now with gui_meas.c
+- eval_string_var should be merged into eval_var and the result should be a
+  struct num (?) that can contain both types. This also means changing all the
+  ops to handle/reject strings.
 
 Open decisions:
 - Q: should loop be (start, last) or (start, iterations) ? or start ... last ?