Don't install the desktop-integration files in our (Wireshark's) data

Don't install the desktop-integration files in our (Wireshark's) data
directory (which is based on our installation prefix): there's no guarantee
that the desktop also uses the same prefix.

Add a note about how we might be able to find the desktop's data directory
should anyone actually run GNOME with a prefix other than /usr .

Also use 'desktop-file-install' to install the desktop file (if configure
found that command).

git-svn-id: http://anonsvn.wireshark.org/wireshark/trunk@47914 f5534014-38df-0310-8fa8-9805f1628bb7