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
Add SPECCTRA dsn export and session file (.ses) import.

Original patch by Josh Jordan and Dan McMahill.

Signed-off-by: Bert Timmerman <bert.timmerman@xs4all.nl>
Added documentation for the dsn exporter.
Added a copyright notice and license text.
Added encapsulation with quotes for netnames, components and pins/pads.

Unconditional encapsulation with quotes is now identical with dsn output of
FreePCB Ver 1.359 (translated by: FpcROUTE Ver 1.303R) and
CadSoft (EAGLE Version 5.10.0)

<todo>
Encapsulation could also be done conditional if the string contains a
dash "-" as this is done in Kicad's PCBNEW (20090216-final)
</todo>
Corrections on Ptr types.

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