Added AppImage structure

misc/dist/appimage/AppRun

+#!/bin/sh
+HERE="$(dirname "$(readlink -f "${0}")")"
+"${HERE}"/godot $@

misc/dist/appimage/godot.desktop

+[Desktop Entry]
+Name=Godot Engine
+GenericName=Libre game engine
+Comment=Multi-platform 2D and 3D game engine with a feature rich editor
+Exec=godot -pm
+Icon=godot
+Terminal=false
+Type=Application
+Categories=Development;IDE;