Updated to 2.3, and generally cleaned up a little

This is probably a bit overdue, but I made a few changes:

• The GML is now updated for 2.3. Consequently, a few changes made to make use of the nice new language features. To you, the main change is that you don’t have to call the init function anymore, since it’s now called automatically. (This was technically possible in 2.2, but I preferred not to.)

• The flush() code is called automatically, and you no longer have to. I can’t imagine this will cause any issues, and previously if you forgot to do it yourself the game would maintain the list of dropped files even after they stop being relevant.

• Added a versioning function, since I don’t have a lot going on in my life and that makes me feel like a real software engineer.

• I wrote some documentation, not that this extension needs much of it.

This is probably the last I’m going to be updating this, unless GameMaker somehow breaks DLL linkage in the future for some reason, or something.