Provide AppImage files

[piegamesde]

A constant pain point is that people have trouble getting a correct Java environment to run BlockMap (it is kind of hard, to be fair). A good solution to this would be to provide AppImage files. While they will be bulky (since the whole point is that they come with a JDK bundled), they'll have the advantage of being "click and run" without further setup.

I have never used AppImage though, and no idea how to create those.

Also, we'd need to investigate if there are similar solutions for Windows users. (Maybe PortableApps?)