Do releases

The lack of formal releases is [cited](https://twitter.com/Det_Conan_Kudo/status/1036395262166622208) as a reason for poor support of the AppImage format in distributions (like Fedora), desktops (like GNOME), and IDEs (like GNOME Builder).

> You guys have made far too many choices favoring agility without any security.

Hence, we should do formal releases.

How can we do so in a way that does not create too much additional administrative work for us?

* Do time-based (e.g., monthly) releases
* Fully automate the process
* Use a versioning scheme that doesn't break (or approximates) semver (e.g., `3.0.$DATE`) (1=klik, 2=klik2/cmg, 3=AppImage)

What should we formally release?
* libappimage
* appimagetool
* linuxdeploy(qt)
* libappimageupdate

cc @Conan-Kudo