Add missing AppImage build files

contrib/AppImage/AppRun

@@ -0,0 +1,9 @@
+#!/bin/bash
+
+HERE="$(dirname "$(readlink -f "${0}")")"
+echo $HERE
+
+export GST_PLUGIN_SCANNER=$(readlink -f $(dirname "$0")/usr/lib/gst-plugin-scanner)
+export GST_PLUGIN_SYSTEM_PATH=$(readlink -f $(dirname "$0")/usr/lib)
+
+${APPDIR}/usr/bin/feather $*

contrib/AppImage/gst-plugin-scanner.sh

@@ -0,0 +1,17 @@
+#!/bin/sh
+
+# Wrapper to launch gst-plugin-scanner inside the AppImage.
+
+HERE="$(dirname "$(readlink -f "${0}")")"
+export PATH="${HERE}:${PATH}"
+
+binary=$(find "$HERE" -name "gst-plugin-scanner-x86_64" | head -n 1)
+LD_LINUX=$(find "$HERE/../../../" -name 'ld-*.so.*' | head -n 1)
+if [ -e "$LD_LINUX" ] ; then
+  case $line in
+    "ld-linux"*) exec "${LD_LINUX}" --inhibit-cache "${binary}" "$@" ;;
+    *) exec "${LD_LINUX}" "${binary}" "$@" ;;
+  esac
+else
+  exec "${binary}" "$@"
+fi