Provide AppImage for Linux

Many Linux users download Arduino directly from the Arduino website rather use what comes with the Linux distribution, since the upstream-provided version is up-to-date while the version in the distributions (especially in stable/LTS versions) is outdated more often than not.

Providing an [AppImage](http://appimage.org/) would have, among others, these advantages:
- Works for most Linux distributions (including Ubuntu, Fedora, openSUSE, CentOS, elementaryOS, Linux Mint, and others)
- One app = one file = super simple for users: just download one AppImage file, [make it executable](http://discourse.appimage.org/t/how-to-make-an-appimage-executable/80), and run
- No unpacking or installation necessary
- No root needed
- No system libraries changed
- Just one format for all major distributions
- Works out of the box, no installation of runtimes needed
- Optional(!) desktop integration with `appimaged`
- Binary delta updates, e.g., for continuous builds (only download the binary diff) using AppImageUpdate
- Can GPG2-sign your AppImages (inside the file)

[Here is an overview](https://github.com/probonopd/AppImageKit/wiki/AppImages) of projects that are already distributing upstream-provided, official AppImages.

Do you think this would be useful?
I'd be happy to help producing an official Arduino AppImage.

The issue names no files, tests, or entry points. Start by locating the Arduino IDE's existing Linux release packaging process and determine how an official AppImage would be produced; done means a documented, distributable AppImage is available for Linux users.

Written by the indexing model from the issue text.