Grayout "Help/Getting started with KiCad" if there is no manual present or redirect to online manual (lp:#1772257)

Original report created by Thomas Pointhuber (pointhi)

# Current Behaviour

1. click on "Help" -> "Pcbnew Manual" when there is no offline manual
2. get "Help file "pcbnew" could not be found."

# Expected Behaviour

gray out items which would not work or fallback with a redirect to the
online reference (for example add a link in the help message)

# System informations

Application: kicad
Version: (5.0.0-rc2-dev-769-g8f6ae7040), debug build
Libraries:
    wxWidgets 3.1.1
    libcurl/7.60.0 OpenSSL/1.1.0h zlib/1.2.11 libidn2/2.0.4 libpsl/0.20.2
(+libidn2/2.0.4) nghttp2/1.31.1
Platform: Linux 4.16.9-1-ARCH x86_64, 64 bit, Little endian, wxGTK
Build Info:
    wxWidgets: 3.1.1 (wchar_t,wx containers) GTK+ 2.24
    Boost: 1.66.0
    OpenCASCADE Community Edition: 6.9.1
    Curl: 7.60.0
    Compiler: Clang 6.0.0 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

Original tags: starter