Packaging: Add initial support for AppImage.

Packaging: Add initial support for AppImage.

Add an "appimage" target that will create an AppImage package. Current
AppImage tools assume that you only have one executable, so add
a custom AppRun wrapper that will let you run our associated CLI
utilities via symlinks, e.g.

    ln -s ./Wireshark-3.2.1-x86.appimage capinfos
    ./capinfos --help

Packaging requires both linuxdeployqt and appimagetool, although we
might be able to reduce this to just linuxdeployqt:

    https://github.com/probonopd/linuxdeployqt
    https://github.com/AppImage/AppImageKit

I haven't done much testing beyond running Wireshark and
capinfos. There are undoubtedly issues that need to be fixed.

Bug: 14464
Change-Id: Ic004ba1962e6a8630ebb017349d9b2c0462fd5fe
Reviewed-on: https://code.wireshark.org/review/30953
Reviewed-by: Gerald Combs <gerald@wireshark.org>