Better handling of disabled layers when editing a footprint (value, ref) from the board editor (avoid the "illegal layer" message box when possible).

Previously, only the lib named "power" was used (and therefore power parts defined in other libs not shown).
Now, all power parts can be loaded from any library.
Also, when loading power parts from the chose component dialog or the lib viewer, libs and parts are filtered:  only power parts and libs containing power parts are listed.
Hotkeys: move a few global strings in the files where ther are actually used (and make them local.) and code cleaning.
Fix a few other minor issues.

Fix some issues in filling zones normals.
Fix an issue with some models that have materials but didn't defined the diffuse color.
Workaround for Bug #1443431.
Implement some missing "code logic" for pervertexperface normals.
Remove some not used functions.
Calculate normals using double type.

3D viewer: minor fixes. Use our class REPORTER to show warning messages and activity info when building the 3D board view. Add a few comments in sources.
Very minor other fixes.

3D viewer: fix (the fix is not perfect) a minor issue in zone rendering: copper thickness was not good when the option 'Show Holes in Zones" was OFF, and "Show Copper Thickness" ON.
Fix a few coverity minor warnings.

board_items_to_polygon_shape_transform.cpp: remove full duplicate code between DRAWSEGMENT and  EDGE_MODULE, and fix incorrect conversion of DRAWSEGMENT polygon type to its polygonal shape.
Gerbview: better info from X2 filefunction attribute displayed in layer manager about copper layers

Pcbnew: always save the line width of footprint graphic edges in files, to allow width = 0 in polygon outlines.
(should not change anything currently for all existing footprints).

Eeschema: extend max number of units per package to 52 and clean code to easily extend this value upto 26x26  (but 52 is a reasonable max value).
Fix a few minor coverity warnings.
Add about_dialog typo fix from Nick Østergaard

Modview: fix issue about mouse on wrong monitor when starting Modview.

Pcbnew: duplicate and create array: popup menus in legacy mode: remove these options for a few items which cannot be (safely)  duplicated (pads in board editor, and a few others )
Modedit, GAL mode: fix a crash when a duplicated area includes the ref or value fields.
Add missing bitmap for pad enumerate.
fix bad flag for Portuguese language.
Fix a few typo in messages.

Fix minor 3D viewer issues (includling change number of segments and its optimization to draw pad holes.)
Fix Bug #1439132 (track and via list on toolbar not always refresh after loading a board).
Remove useless tool in Modedit.

3d viewer: fix a few warning coverity (and a potential bug). fp libs wizard: make it internationalized, and replace an untranslatable string created by grouping 3 strings by one translatable string.

* Create new dialog to edit environment variables.
* Add helper class ENV_VAR_ITEM to manage environment variable values and
  if they were defined externally.
* A path configuration dialog access to KiCad, CvPcb, Pcbnew, and Footprint Editor
  window menus.
* Add validator for environment variables.

* Make OK button the default action in the 3D viewer option dialog per UI
  policy.
* Fix standard button sizer padding in 3D viewer options dialog.
* Let sizers do their job by using default size for 3D viewer options
  dialog.
* Set 3D viewer hot key to Alt+3 in all parent windows where 3D viewer is
  accessible.
* Minor code cleaning and coding policy fixes.

Minor fix: Gerber X2 attribute: remove one attribute (TF .GerberVersion, now no more in use) I added in my recent changes relative to the new J5 gerber version attributes.