* Add code to test if any of the components in the schematic cache have been changed
  in the component libraries.
* Prompt user to accept or reject components when changes are found.
* If the user chooses to use the components in the cache, create a new library with the old
  components and add it to the beginning of the component library list so the schematic will
  not be changed.
* Create dialogs to handle user feedback and status.

Dialog layers setup: cosmetic enhancements (fix a very minor issue on Windows). Remove a not used and outdated file.

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.

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.

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: 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.

* 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.

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.

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)

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)

class PLOTTER: dash lines parameters in one place for plot. I made minor changes (values in mm and set to give the same size as dashed lines drawn by wxDC, using short dash.)

Fix a few minor coverity warnings.