Minor fixes: dialog rescue: make strings translatable. Remove a few coverity warnings (not initialized members). Very minor other changes.

* 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.

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.

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

* Remove redundant definition of KICAD_DATA_PATH since DEFAULT_INSTALL_PATH
  was already defined.
* Use DEFAULT_INSTALL_PATH as the substitution for the default environment
  variable paths.
* Add option to define DEFAULT_INSTALL_PATH during CMake configuration with
  fall back to CMAKE_INSTALL_PREFIX if not not defined.
* Developer note: this change is primarily for package builders where
  CMAKE_INSTALL_PATH is the path where the package builder temporarily
  installs the build output before creating the package.  When building
  packages, DEFAULT_INSTALL_PATH should be set to the path where the
  package will be installed by default.

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

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.

* Fix bug (and Coverity error) when deleting environment variable map entries.
* Remove debug logging code inadvertently left over from development.

* 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.

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.

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)