Generate and upload AppImage

[probonopd]

This PR, when merged, will compile this application on Travis CI upon each git push, and upload an AppImage to your GitHub Releases page.

Providing an AppImage would have, among others, these advantages:

• Applications packaged as an AppImage can run on many 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
• Works out of the box, no installation of runtimes needed
• Optional desktop integration with appimaged
• Optional binary delta updates, e.g., for continuous builds (only download the binary diff) using AppImageUpdate
• Can optionally GPG2-sign your AppImages (inside the file)
• Works on Live ISOs
• Can use the same AppImages when dual-booting multiple distributions
• Can be listed in the AppImageHub central directory of available AppImages
• Can double as a self-extracting compressed archive with the --appimage-extract parameter
• No repositories needed. Suitable/optimized for air-gapped (offline) machines
• Decentralized

Here is an overview of projects that are already distributing upstream-provided, official AppImages.

PLEASE NOTE: For this to work, you need to set up GITHUB_TOKEN in Travis CI for this to work; please see https://github.com/probonopd/uploadtool.

If you have questions, AppImage developers are on #AppImage on irc.freenode.net.

[mcejp]

Hi, I have ported your work to GitHub Actions: https://github.com/mcejp/wxHexEditor/tree/feature/CI-AppImage

It doesn't push a GitHub release though, just an artifact.

[EUA]

Hello. Thanks for your work.
I am not against AppImage, but create AppImage for each git push is... something that I do not like.
Because that binary might be broken and could create some damage on file integrity...
Better to release AppImage for releases.

[probonopd]

@EUA maybe a good way would be to build the AppImage for each build (it can be helpful for developers and testers) but not upload it to GitHub Releases (only to GitHub Artefacts)?

[EUA]

@probonopd Yup. I will try to make AppImage when I make release. But first I need to learn how to do it.
For this merge, I need to learn Travis-CI first.

[probonopd]

At this time I'd really recommend to use GitHub Actions instead of Travis CI.

Would you be interested in a pull request that does it?