Provide an AppImage for Linux

[probonopd]

AppImage is a format for portable applications on Linux that bundles an application and everything it needs to run that cannot be expected to be part of the base system (e.g., Qt). The resulting AppImage runs on most not-too-outdated Linux desktop distributions - without installation, without root, without touching any of the system libraries.

It also runs easily from a Live USB or Live ISO without the need for installation - just download, set the executable bit (via the GUI or with chmod a+x, and run.

It would be useful to have an official AppImage from the EncryptPad project.
I have made an example AppImage here:
https://bintray.com/probono/AppImages/EncryptPad/_latestVersion#files

The recipe that generated this AppImage is here:
https://github.com/probonopd/AppImages/blob/master/recipes/meta/EncryptPad.yml

[evpo]

I have created the qtdeployment branch with changes to configure.sh and a new linux_deployment directory with a script and static AppDir files to automate the process. It currently generates an AppImage. I have tested it on Ubuntu and Fedora. The next release will have an official AppImage.

[probonopd]

See https://github.com/probonopd/uploadtool for an easy way to upload continuous build artefacts from Travis CI to GitHub Releases.