* Enhance class PLUGIN's comments to improve their value as instructional.
  * Switch #includes from "file.h" to <file.h>, and fiddle with search paths
    by using include_directories(BEFORE ...)
  * dialog_page_settings did not have symmetrical space on its right border.
    And it now remembers it last position and size within a program session.
  * base_screen.cpp is now compiled twice, once for {pcbnew,cvpcb} and once
    for eeschema, in preparation for "compile time" support of internal units
    rather than "runtime" support.

compile base_screen.cpp separately for {pcbnew,cvpcb} and eeschema, so we can diverge internal units at compile time, not runtime

Pcbnew: 2 minor enhancements

Pcbnew: fix a serious bug: crashes when attempt to modify a zone that had lost its net (after eeschema changes for instance)

keep all the labelling tools together in the eeschema toolbar.

portrait setting was being overwritten in dialog_page_settings.cpp, add updatePortrait(), improve comments

    sizes.  Tested with postscript output only.  Required minor file format changes
    to reflect the "portrait" setting.  common/dialogs/dialog_page_settings.cpp
    uses a checkbox but its name is "Landscape", which is inverted from portrait,
    but since it is the more common choice, I used that rather than portrait.
    The tooltip for that checkbox makes it clear.  No portrait mode is supported
    for "User" paper size.

* Remove unnecessary copy constructors from board and module library
  objects.
* Add doClone() method to board and library objects.
* Add comment to class definitions where the default copy constructor
  generated by the compiler was adequate.
* Replace copy method with clone method where applicable.
* Remove DuplicateStruct() function.
* Remove track object copy function.

Eeschema: Fix erroneous message [Read Only] after creating a new project, and obviously the corresponding schematic file does not yet exist (replaced by [no file])

Gebview: fix issue in export to pcbnew (Track arc shapes are now exported, approximated by segments)

Pcbnew: fix issue when KICAD_NANOMETER is defined: when zones use htcth to show zones areas, hatch lines were incorrectly calculated (hunded of thousand lines created)
Eeschema: fix issue in search: search not made in reference strings.
Minor fixes, code cleaning and comment enhancements.

* Remove unnecessary copy constructors from schematic and component
  library objects.
* Add comment to class definitions where the default copy constructor
  generated by the compiler was adequate.
* Add assignment operator to EDA_ITEM, SCH_ITEM, and all schematic
  objects where the default assignment operator generated by the
  compiler would not be adequate.