DXF export should export dimension items as DXF dimensions (lp:#1832717)

Original report created by Seth Hillbrand (sethh)

When plotting DXF files from pcbnew, dimension items are output as graphic
lines.  The DXF Specification[1] shows how to specify a dimension object
(Ch 6, Pg 78), which would be more useful in the final DXF file.


[1] https://images.autodesk.com/adsk/files/autocad_2012_pdf_dxf-
reference_enu.pdf

Application: Pcbnew
Version: (5.1.0-727-g66a43612b), debug build
Libraries:
    wxWidgets 3.0.4
    libcurl/7.54.0 LibreSSL/2.6.4 zlib/1.2.11 nghttp2/1.24.1
Platform: Mac OS X (Darwin 18.2.0 x86_64), 64 bit, Little endian, wxMac
Build Info:
    wxWidgets: 3.0.4 (wchar_t,STL containers,compatible with 2.8)
    Boost: 1.69.0
    OpenCASCADE Community Edition: 6.9.1
    Curl: 7.54.0
    Compiler: Clang 9.0.0 with C++ ABI 1002

Build settings:
    USE_WX_GRAPHICS_CONTEXT=ON
    USE_WX_OVERLAY=ON
    KICAD_SCRIPTING=ON
    KICAD_SCRIPTING_MODULES=ON
    KICAD_SCRIPTING_PYTHON3=OFF
    KICAD_SCRIPTING_WXPYTHON=ON
    KICAD_SCRIPTING_WXPYTHON_PHOENIX=OFF
    KICAD_SCRIPTING_ACTION_MENU=ON
    BUILD_GITHUB_PLUGIN=ON
    KICAD_USE_OCE=ON
    KICAD_USE_OCC=OFF
    KICAD_SPICE=ON

Original tags: dxf export pcbnew