Pcbnew: more about keepout areas: remove keepout for pads option and add keepout for copper zones option.
Export keepout to Specctra DSN file.

This is *a work in progress*, so some features are missing, and/or could be modified.
Mainly keepout zones are not yet exported to autorouters, and pads are not taken in account.
Some code cleanup in polygon.*

Fix incorrect import of smd pads from Gpcb.
Very minor change in dialog print.

* Use index when saving layers to handle translated layer names.
* Add LOCALE_IO switcher when loading s-expression format.
* Make SEGVIA default to through hole to improve file size.
* Don't save default orientation of 0 degrees in TEXTE_MODULE and TEXTE_PCB.
* Don't save default zone configuration parameters.
* Save multiple zone corners on a single line.
* Fixed missing ( check in graphic and module arc parsers.
* Consistency improvements for DRAWSEGMENT and EDGE_MODULE objects.
* Minor tweaks to D_PAD formatting.

* Change save order of drawings to match legacy file format object order.
* Set end point of SEGVIA to start point so the round trip back to legacy
  format is the same.
* Fixed bug saving and loading oval pad drill parameters.
* Fixed bug saving and loading rectangular pad deltas.
* Fixed missing layer for PCB_TARGET.
* Fixed EDA_TEXT italic bug.  Thanks Dick!
* Remove some left over debug logging statements.

* Apply Dick's patch along with a few minor changes to fix some clashes with
  changes in my branch.  Thanks Dick!
* Added missing DRAWSEGMENT line angle.
* Fix thru hole pad layer mask issue.

* Add s-expression file parser object and keyword files.
* Fix minor issues with s-expression file formatting.
* Fix a minor bug the zone container fill state parsing in the legacy plugin.
* Move EDA_TEXT visibility definition to eda_text.h.
* Add minor BOARD_ITEM object improvements to support s-expression file
  parser.