Generate AppImage for Linux

[probonopd]

This patch, when merged, will build continuous builds for Linux and bundle them as an AppImage on Travis CI (need to enable Travis CI for the repository like shown here).

Providing an AppImage 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, 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 of projects that are already distributing upstream-provided, official AppImages.

[probonopd]

Thanks for merging.

Instead of storing AppImage builds temporarily for 14 days each on transfer.sh (where no one finds them because the URL is hidden in the build log), you could use GitHub Releases to store the binaries permanently. This way, they would be visible on the Releases page of your project. This is what I recommend. See https://docs.travis-ci.com/user/deployment/releases/. If you want to do this for continuous builds, also see https://github.com/probonopd/uploadtool.