Relocate user data directory to ~/.local/share/ on Linux

I found the user data directory at `~/nodebox` kind of annoying. Placing the directory in `~/.local/share/` doesn't pollute the user's home directory as much and is more similar to the approach on Windows and OS X.

I also deleted `PlatformUtils.java` as it is redundant given `Platform.java`. That said, it kind of makes sense for the platform functionality to be located in `client` rather than `ui`, so it might be worth moving Platform and refactoring the code that depends on it.