Technical layer draw order on Modern toolset (lp:#1776369)

Original report created by Fabián Inostroza (fabianinostroza)

With the Legacy toolset the technical layers were drawn on top of copper
layers (or some kind of xor), with the modern toolset the order is not well
defined.

As an example, the F.CrtYd layer is drawn over F.Cu only after selecting
the F.CrtYd layer, selecting other layer keeps the F.CrtYd layer on top of
F.Cu.
The B.CrtYd layer is always drawn on top of the B.Cu layer, no need to
select B.CrtYd first.

F.SilkS, B.SilkS, Dwgs.User (used to define specify keepout areas in
footprints) are drawn behind copper layer unless they are selected.

I don't know if it is a good idea to always draw the silkscreen layers on
top of copper layers but I think that the CrtYd and Dwgs.User layers
should, as they help positioning the components on the board and to define
the real keepout areas.

Application: kicad
Version: (5.0.0-rc2-119-g896ad3774), debug build
Libraries:
    wxWidgets 3.0.4
    libcurl/7.60.0 GnuTLS/3.5.18 zlib/1.2.11 libidn2/2.0.4 libpsl/0.20.2
(+libidn2/2.0.4) libssh2/1.8.0 nghttp2/1.32.0 librtmp/2.3
Platform: Linux 4.16.0-2-amd64 x86_64, 64 bit, Little endian, wxGTK
Build Info:
    wxWidgets: 3.0.4 (wchar_t,wx containers,compatible with 2.8) GTK+ 2.24
    Boost: 1.62.0
    OpenCASCADE Community Edition: 6.9.1
    Curl: 7.60.0
    Compiler: GCC 7.3.0 with C++ ABI 1011

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

Original tags: pcbnew