Improve the db-seed-i18n.py -script to actually grab all strings.

Previously the script could only handle oils_i18n_gettext -markers
if the marker and it's parameters were all on one line.

Now it also correctly parses the postgres E'' escaped strings, and
removes sql string concatenations.

Also adds test cases.

Signed-off-by: Pasi Kallinen <pasi.kallinen@pttk.fi>
Signed-off-by: Dan Scott <dscott@laurentian.ca>