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

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.

cc @zdenop @marathone

[probonopd]

An AppImage for testing is available at https://github.com/probonopd/scantailor-advanced/releases.

[4lex4]

Rejected due to #69, #124

Resume:

1. Qt theming is broken if AppImage is launched in an environment different from that it was built on.
2. The same for Qt High DPI displays support.

[probonopd]

Then we should fix those things, shouldn't we @4lex4? ;-)

Qt theming is broken if AppImage is launched in an environment different from that it was built on.

I'd say: Cosmetic, hardly a reason not to offer an AppImage at all.

But anyhow:
With some fine-tuning, it can probably be made work if we know what "broken" exactly is referring to. For example, the Scribus (also Qt based) AppImage theming looks very native on a Xubuntu (Gtk) system.

If you want KDE theming, then as I wrote in #69 probably some plugin for KDE theming would need to be bundled inside the AppImage.

The same for Qt High DPI displays support

Please elaborate. In #124 someone says that the fonts are "too small" in some old build of the AppImage.

I wonder whether e.g., the Scribus AppImage is exhibiting the same issues, as it is also a Qt-based applications used by many design-affine users every day. @aoloe do those concerns sound familiar to you?

[aoloe]

i have the feeling that the appimage behaves correctly in an hdpi setting.

but since i have custom settings in place to run the normal scribus i think that i'm not the right one to judge about it.