pcbnew: via dialog, show interesting nets at top

[Thomas Pointhuber]

When someone want's to edit the net of a via, it's preferably that net's the via can actually connect to are displayed on top (probably seperated by some horizontal bar)

Application: pcbnew
Version: (2018-01-05 revision 1e1c12301)-3d_glitch, release build
Libraries:
    wxWidgets 3.0.3
    libcurl/7.57.0 OpenSSL/1.1.0g zlib/1.2.11 libidn2/2.0.4 libpsl/0.19.1 (+libidn2/2.0.4) libssh2/1.8.0 nghttp2/1.29.0
Platform: Linux 4.14.11-1-ARCH x86_64, 64 bit, Little endian, wxGTK
Build Info:
    wxWidgets: 3.0.3 (wchar_t,wx containers,compatible with 2.8) GTK+ 2.24
    Boost: 1.65.1
    Curl: 7.57.0
    Compiler: GCC 7.2.1 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=OFF
    BUILD_GITHUB_PLUGIN=ON
    KICAD_USE_OCE=OFF
    KICAD_SPICE=ON

[Jeff Young (jeyjey)]

There's now a filter in the Nets combobox dropdown.