AppImage support

[mherrmann]

fbs currently lets you create .deb, .rpm and .pkg.tar.xz packages for Ubuntu, Fedora and Arch Linux, respectively. People also often ask about support for creating AppImages.

My goal is for fbs to follow the same strategy as Linus Torvalds' Subsurface: Offer native packages for the main Linux distributions, and AppImages for users who prefer them or are on less common distributions.

The goal of this issue is thus to add support for AppImages to fbs. I am creating it not because this will likely be implemented soon (there are other, more important, tasks right now, such as code signing and automatic updates on Windows and Mac). But to document the associated considerations, and that AppImages have not been overlooked.

For previous discussions of this topic, please see #44 and #47. Especially in the latter, @probonopd did a great job of outlining the advantages of AppImages.

[probonopd]

Maybe you are interested in https://github.com/niess/python-appimage#for-applications-developers.
It seems to be a very straightforward way to bundle a Python based application along with Python for Linux.