* glom/libglom/data_structure/field.[h|cc]: to_file_format(),
from_file_format() documentation: Mention that this uses CSV format.
from_file_format(): Unescape double quotes, because we escape quotes
as that in to_file_format().
This avoids reading them as double quotes when opening an example file.
This will also correct the interpretation of text default values.
* Makefile_tests.am:
* tests/test_field_file_format.cc: Add a test of this for text and image
data, checking that what we write is what we read.