Minor fixes: make a message translatable in CvPcb. Dialog choose component: fix a missing min size calculation and make min size bigger.

* 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.

3D-viewer: Fixes and enhancements (add 3D bounding box for components). Add comments in header files. Fix some coding style issues.

* 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.

Better error messge when a .kiface file ( or,  if it exists, a .dll or .so shared library ) is not found at start up

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)

they are now shown only when a new board is loaded, if there are warnings.

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.