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.

Pcbnew: Force visibility of items which have no option in dialogs to controle this visibility, when reading the .kicad_brd file.
these options are used in GAL mode, but the usercannot control this visibility. So if the board file does not have these visibility flags set, the GAL never show them.
Fix also a compil warning.

* Add footprint and drop table from the Pcbnew footprint library table menu
  entry to make it clear to new users.
* Add component to the Eeschema library configuration menu entry.

Eeschema enhancements: Option to annotate keeping multi-unit parts grouped and friendlier warning for modifying power values.

Very minor fixes: 3D viewer: remove artifacts with NPTH pads having the save size as their hole. Kicad: remove duplicate submenu in menubar. remove a minor coverity warning

Make hotkey string info translatable in dialogs. These strings are now prefixed by _HKI , which can be used by translation tools to extract them.

Fix Bug #1426675 (Undo and cancel commands don't have effect for Duplicate operation in legacy mode)

Plot pdf: fix issue when a 0 sized text is plotted (files are broken): now 0 sized texts are ignored.
pcbnew: remove a false error message when undoing a change on a text, or some other board items (graphic items for instance)

Try to fix Bug #1431765 (Pcbnew crashes in GAL when entering 3d display when a board outline is not closed)
This change modify the way warning messages are displayed: now the are displayed outside a paint event.

Cvpcb: Shows a better message when a netlist is not found.

Try to fix Bug #1431391 (Unable to edit pins in Schematic library editor). It happens on XFCE, but also on Unity. This fix works on Unity. (it is due to the fact the dialog is not raised, for an unknown reason, and this bug depend on the way Eeschema and Libedit are run).
Add Ctrl+E hotkey in eeschema to edit a component with Libedit (similar to pcbnew Ctrl+E hotkey)
As usual, fix a few minor coverity warnings

Remove a few coverity warnings. Fix some comments which were incorrect, due to the changes in code. Remove not compiled code (kept as comments), now outdated or useless
legacy plugin: better compatibility with old brd files (current track width and current vias size, stored in SETUP section, were not read from file and incorrectly set)

Eechema, libedit, pin dialog editor: show pin texts  in pin  preview canvas.

Fix minor bug #1429990 (3D viewer shows hidden footprint graphic texts ). Fix minor coverity warnings (not initialized members and dead code)

Fix a few minor coverity warnings.

Moved "Place Target" and "Place Module" tools from DRAWING_TOOL to PCB_EDITOR_CONTROL (to avoid placing modules in the footprint editor).