Update for 1.13 (kind of)

[piegamesde]

So I started updating the codebase to be able to load 1.13 worlds and, that might have escalated quite a bit. Through the process I rewrote almost every single line of code, even if I kept the rough structure. I split the repository into four modules and converted it to Gradle because Maven didn't want to. (If you are wondering, I started a fresh repository and migrated the code gradually and copied everything back for this pull request).

A lot of the features got removed and not added back in yet, and the Eclipse project is kind of broken again, but calling something like ./gradlew -Dorg.gradle.java.home=/usr/lib/jvm/java-8-openjdk :BlockMap-standalone:shadowJar spits out a fat jar that might render worlds with a GUI :) . I also renamed the project so this should be reverted before merging.

@TOGoS I don't expect you to merge this big mess. This will require a lot of additional work and I probably won't do it. Unless you are absolutely willing to get this merged, I will continue this in my own repository and perhaps mirror it on this branch.

Fixes #39