Add AppImage packaging for Linux.

packaging/linux/AppRun.in

@@ -0,0 +1,35 @@
+#!/bin/sh
+
+# Make sure the user has a sufficiently recent version of mono on the PATH
+MINIMUM_MONO_VERSION="4.2"
+
+make_version() {
+  echo "$1" | tr '.' '\n' | head -n 4 | xargs printf "%03d%03d%03d%03d";
+}
+
+mono_missing_or_old() {
+  command -v mono >/dev/null 2>&1 || return 0
+  MONO_VERSION=$(mono --version | head -n1 | cut -d' ' -f5)
+  [ "$(make_version "${MONO_VERSION}")" -lt "$(make_version "${MINIMUM_MONO_VERSION}")" ] && return 0
+  return 1
+}
+
+if mono_missing_or_old; then
+  if command -v zenity > /dev/null; then
+    zenity --no-wrap --error --title "{MODNAME}" --text "{MODNAME} requires Mono ${MINIMUM_MONO_VERSION} or greater.\nPlease install Mono using your system package manager."
+  else
+    printf "{MODNAME} requires Mono %s or greater.\nPlease install Mono using your system package manager.\n" "${MINIMUM_MONO_VERSION}"
+  fi
+  exit 1
+fi
+
+# Run the game or server
+HERE="$(dirname "$(readlink -f "${0}")")"
+export PATH="${HERE}"/usr/bin/:"${PATH}"
+export XDG_DATA_DIRS="${HERE}"/usr/share/:"${XDG_DATA_DIRS}"
+
+if [ -n "$1" ] && [ "$1" = "--server" ]; then
+  exec "openra-{MODID}-server" "$@"
+else
+  exec "openra-{MODID}" "$@"
+fi