* 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.