Add AppImage for easier Linux distribution

[TequinDragon]

Hi, I was trying to give ngscope a try and was a bit frustrated with having to move files into ideal locations just to run the application.

I'm sure there will be a lot of others that will experience similar frustration if they are not wanting to or unable to use the provided deb/rpm packages. For example, Manjaro or Arch users

I'm creating this issue as I don't have the time to create a pull-request myself, but my hopes is that someone can pick it up and add it to the build pipeline.

An AppImage is a self-contained Linux application that just runs without needing to install or movie files into appropriate directories. It would be very welcoming to have this included in the distributed assets in releases.

1. Create a directory that looks like this

   AppRun
   ngscopeclient.desktop
   ngscopeclient.png --(symbolic link)--> usr/share/ngscopeclient/icons/128x128/app-icon.png
   usr/
   ├─ bin/
   │  ├─ ngscopeclient
   ├─ lib/
   │  ├─ libscopehal.so
   │  ├─ libscopeprotocols.so
   ├─ share/
   │  ├─ ngscopeclient/
   │  │  ├─ icons/
   │  │  ├─ shaders/
   

2. AppRun file contents

   #!/bin/bash
   
   export LD_LIBRARY_PATH="${APPDIR}/usr/lib:${LD_LIBRARY_PATH}"
   
   ${APPDIR}/usr/bin/ngscopeclient

3. ngscopeclient.desktop

   [Desktop Entry]
   Name=NgScopeClient
   Exec=ngscopeclient
   Icon=ngscopeclient
   Type=Application
   Categories=Utility;

4. Then use a tool like appimagetool to turn the directory into a AppImage

   ARCH=x86_64 ./appimagetool-x86_64.AppImage ngscopeclient ngscopeclient.AppImage

The resulting ngscopeclient.AppImage can be run as is from either the command line or file manager.