AppImage build: Include libxcb into the image

Ubuntu 20.04 needs `libxcb-xinerama.so.0`
and Debian 10 needs `libxcb-util.so.1` for the Qt `xcb` plugin.

-----

ported from 380f04a805

see https://github.com/Electron-Cash/Electron-Cash/issues/2196

-----

In particular, see this comment:

>> I confirmed that both Ubuntu 20.04 and Debian 10.8 clean install have an issue with the 4.2.4 AppImage.
>
> Any idea what broke it? I assume it used to work.

Yes, it did work in 4.2.3 which used PyQt5 5.13.2. I just had a look at the xcb plugin and it definitely has less dependencies:
```
ago@ubuntu2004vm:~/src/Electron-Cash/dist$ ldd squashfs-root/usr/lib/python3.6/site-packages/PyQt5/Qt/plugins/platforms/libqxcb.so|grep xcb
        libX11-xcb.so.1 => /lib/x86_64-linux-gnu/libX11-xcb.so.1 (0x00007f0ec9d07000)
        libxcb.so.1 => /lib/x86_64-linux-gnu/libxcb.so.1 (0x00007f0ec9cdd000)
        libxcb-xkb.so.1 => /lib/x86_64-linux-gnu/libxcb-xkb.so.1 (0x00007f0ec6ead000)
```

compared to PyQt5 5.15.2 in 4.2.4:
```
ago@ubuntu2004vm:~/src/Electron-Cash/dist$ ldd squashfs-root/usr/lib/python3.8/site-packages/PyQt5/Qt/plugins/platforms/libqxcb.so|grep xcb
        libX11-xcb.so.1 => /lib/x86_64-linux-gnu/libX11-xcb.so.1 (0x00007fcb5715f000)
        libxcb-icccm.so.4 => /lib/x86_64-linux-gnu/libxcb-icccm.so.4 (0x00007fcb57158000)
        libxcb-image.so.0 => /lib/x86_64-linux-gnu/libxcb-image.so.0 (0x00007fcb56f51000)
        libxcb-shm.so.0 => /lib/x86_64-linux-gnu/libxcb-shm.so.0 (0x00007fcb56f4c000)
        libxcb-util.so.1 => /lib/x86_64-linux-gnu/libxcb-util.so.1 (0x00007fcb56d46000)
        libxcb-keysyms.so.1 => /lib/x86_64-linux-gnu/libxcb-keysyms.so.1 (0x00007fcb56d41000)
        libxcb-randr.so.0 => /lib/x86_64-linux-gnu/libxcb-randr.so.0 (0x00007fcb56d2e000)
        libxcb-render-util.so.0 => /lib/x86_64-linux-gnu/libxcb-render-util.so.0 (0x00007fcb56d27000)
        libxcb-render.so.0 => /lib/x86_64-linux-gnu/libxcb-render.so.0 (0x00007fcb56d16000)
        libxcb-shape.so.0 => /lib/x86_64-linux-gnu/libxcb-shape.so.0 (0x00007fcb56d11000)
        libxcb-sync.so.1 => /lib/x86_64-linux-gnu/libxcb-sync.so.1 (0x00007fcb56d07000)
        libxcb-xfixes.so.0 => /lib/x86_64-linux-gnu/libxcb-xfixes.so.0 (0x00007fcb56cfd000)
        libxcb-xinerama.so.0 => /lib/x86_64-linux-gnu/libxcb-xinerama.so.0 (0x00007fcb56cf8000)
        libxcb-xkb.so.1 => /lib/x86_64-linux-gnu/libxcb-xkb.so.1 (0x00007fcb56cda000)
        libxcb.so.1 => /lib/x86_64-linux-gnu/libxcb.so.1 (0x00007fcb56cae000)
```

contrib/build-linux/appimage/Dockerfile

@@ -28,6 +28,8 @@ RUN apt-get update -q && \
         libzbar0=0.10+doc-10ubuntu1  \
         libdbus-1-3=1.10.6-1ubuntu3.6 \
         libxkbcommon-x11-0=0.5.0-1ubuntu2.1 \
+        libxcb-util1=0.4.0-0ubuntu3 \
+        libxcb-xinerama0=1.11.1-1ubuntu1 \
         libc6-dev=2.23-0ubuntu11.2 \
         && \
     rm -rf /var/lib/apt/lists/* && \

contrib/build-linux/appimage/build.sh

@@ -176,6 +176,8 @@ info "Copying additional libraries"
     cp -f /usr/lib/x86_64-linux-gnu/libusb-1.0.so "$APPDIR/usr/lib/libusb-1.0.so" || fail "Could not copy libusb"
     # some distros lack libxkbcommon-x11
     cp -f /usr/lib/x86_64-linux-gnu/libxkbcommon-x11.so.0 "$APPDIR"/usr/lib/x86_64-linux-gnu || fail "Could not copy libxkbcommon-x11"
+    # some distros lack some libxcb libraries (see https://github.com/Electron-Cash/Electron-Cash/issues/2196)
+    cp -f /usr/lib/x86_64-linux-gnu/libxcb-* "$APPDIR"/usr/lib/x86_64-linux-gnu || fail "Could not copy libxcb"
 )
 
 info "stripping binaries from debug symbols."