GerbView: bug: opening a Gerber Job File followed by Excellon Drill Files causes the NPTH drill file to replace the F.SilkS layer rather than opening up in addition to it (lp:#1782931)

Original report created by Gabriel Staples (ercaguy)

Application: gerbview
Version: 5.0.0-rc3-unknown-101b68b~65~ubuntu14.04.1, release build
Libraries:
    wxWidgets 3.0.2
    libcurl/7.35.0 OpenSSL/1.0.1f zlib/1.2.8 libidn/1.28 librtmp/2.3
Platform: Linux 4.4.0-112-generic x86_64, 64 bit, Little endian, wxGTK
Build Info:
    wxWidgets: 3.0.2 (wchar_t,wx containers,compatible with 2.8) GTK+ 2.24
    Boost: 1.54.0
    OpenCASCADE Community Edition: 6.8.0
    Curl: 7.35.0
    Compiler: GCC 4.8.4 with C++ ABI 1002

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

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From Pcbnew, export Gerbers with a Gerber job file, as well as drill files.

From GerbView, File --> Open Gerber Job file works perfectly. I get 9
layers opened up.
But I want to view the drill files too, so File --> Open Excellon Drill
Files. This causes the NPTH drill file to replace the F.SilkS layer (bug!),
which was the first layer before, whereas the PTH drill file gets added to
the end (good!).

Work-around: manually open up all gerber files + the drill files rather
than opening up the gerbers via the Gerber Job file.
Downside of the work-around: the layers are no longer in physical board
order, like they are when you open up the Gerbers via the job file. :(

Original tags: gerbview