Small set of changes salvaged from PR #1108.

PR #1108 over time became a bit of a messy monster and is now obsolete due to code changes that happened in the meantime. Some changes are probably worth salvaging though. 

In this PR I'm submitting only the smallest set of largely independent and hopefully uncontroversial changes I could find. With GameEngine extending AutoCloseable I'm also paving the way for the changes to the PC Facade discussed in #1108. Only afterwards I'll try and tackle again the broader changes to the source of GameEngine and TerasologyEngine.

In any case I'll aim to submit considerably smaller PRs, in the hope they'll be easier to deal with.