pcbnew: allow column sorting (ordering) in "list nets" dialog (lp:#1838161)

Original report created by Oleg Endo (oleg.endo)

The "list nets" dialog in pcbnew does not support support sorting of the
filtered displayed rows by any of the columns.  That can be extremely
useful when checking groups of traces which need to be length matched, e.g.
sort them by length.  Right now the only option to get that is to export
the result to a CSV file and open it in another tool (excel, calc ..) ...
quite annoying.

Application: KiCad
Version: 6.0.0-unknown-2af41e8~86~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:
    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

Original tags: pcbnew ui