Move library version to version.txt

[cmelchior]

In Realm Java we define the library version in a simple text file in the root called version.txt. This makes a lot of things easier as a lot of build logic depend on having that available.

In Kotlin we are defining it in buildSrc/src/main/kotlin/Config.kt. This makes accessing and editing it slightly annoying.

We should consider moving the constant to a simple file instead, but it turns out to be non-trivial as the Config.kt file is being prebuilt before anything else, and not part of the build script to boot. This makes accessing the environment a bit difficult.

It could probably be accomplished by either auto-generating a .kt file with the information or otherwise manipulate the Config.kt file before it is being built.