pcbnew: plot to dxf option in mm doesnt remember

[Eldar Khayrullin]

Current branch 5.1.
After reopen plot dialog option plot in mm resets to inches.

[Seth Hillbrand (sethh)]

Please add your KiCad version information

[Eldar Khayrullin (eldar)]

Application: KiCad
Version: 5.1.2-br-unknown-02d43be~84~ubuntu18.04.1, release build
Libraries:
    wxWidgets 3.0.4
    libcurl/7.58.0 OpenSSL/1.1.1 zlib/1.2.11 libidn2/2.0.4 libpsl/0.19.1 (+libidn2/2.0.4) nghttp2/1.30.0 librtmp/2.3
Platform: Linux 4.15.0-54-generic x86_64, 64 bit, Little endian, wxGTK
Build Info:
    wxWidgets: 3.0.4 (wchar_t,wx containers,compatible with 2.8) GTK+ 3.22
    Boost: 1.65.1
    OpenCASCADE Community Edition: 6.9.1
    Curl: 7.58.0
    Compiler: GCC 7.4.0 with C++ ABI 1011

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

[KiCad Janitor (kicad-janitor)]

Fixed in revision 4a1edf581e2c2853d1e4df4ad0167c15866bc3b5
https://git.launchpad.net/kicad/patch/?id=4a1edf581e2c2853d1e4df4ad0167c15866bc3b5