hid/gtk: Focus file save dialog at exit to "Save", not "Close without saving".

Avoids inadvertently loosing work when closing the application. "Cancel" would
also be a safe choice of default focus, but the choice of "Save" is copied from
gedit to help give better platform consistency on Gnome systems.
png: add explicit round() calls to the scaling

After this commit the tests pass on both 32- and 64-bit builds.
Hopefully this time it's really dead; the previous "fix" af27c2d
by myself merely increased the DPI of the output, but this one
attempts to get deterministic rounding behaviour.

Tests from folks with actual 32-bit machines are welcome.

Closes-bug: lp-860037
polygon.c: Readability cleanup for frac_circle()
Fix some broken assumptions regarding 32-bit integer coordinates in polygon1.c
Added metric DXF header template file.
Added imperial DXF header template file.
Added src/hid/dxf/hid.conf
New file: src/hid/dxf/dxf.c
Modified configure.ac and src/Makefile.am.
No linetype, color or thickness for DXF BLOCK entity definitions.
Correction of a typo in a comment.
Added a newline where needed.
Added code to dxf_write_header_imperial_new().
Added comments to dxf_write_header_imperial_new().
Use landscape Letter format paper sizes in dxf_write_header_imperial_new().

Was metric landscape A3 format paper sizes.
Added comments to dxf_write_header_metric_new().
Added a README file.
Replace whitespace character with underscore character.
Corrections/additions in the doxygen comments.
Removed the RCSID.

Signed-off-by: Bert Timmerman <bert.timmerman@xs4all.nl>