🛠 Simple editor startup

Now it's much easier to get started with the level editor. It used to take about ten clicks to create and test the first playable custom level, and it was hard to figure out. Now it takes about three clicks, and they're a lot more obvious.

• If there are no custom adventures, the empty slot is selected by default, so the create-adventure button is visible.
• A newly created adventure contains one level by default, instead of being empty.
• Each new level is no longer an infinite expanse of wall. Now there's a little cave, with an entrance and exit, so it's immediately playable, and easier to edit.