Switch to Twison Twine format and enhance the importer

[grahamearley]

This PR migrates the importer from the TwineJson Twine format to the Twison Twine format (in response to #93).

It also allows users to select the destination for their story prefab (instead of just putting it in the Assets/Ignored/ directory. Also, the prefab is now named after the title of the Twine story, instead of just being called Story.prefab.

Here's what the new importer looks like:

There are a few other things to note about this change:

• You no longer use a .json file to import the story -- now you copy and paste the json code. This is because the Twison exporter simply creates a page with all the json text and the easiest way to import it is to copy and paste it, rather than copy and paste it into an intermediary file.
• We now support "alias" child names for children nodes. For example, in one of the complex workhouse Twine stories, all the nodes are titled things like Righteous01 or 6:00am. When those nodes are linked to, though, they use the Twine syntax [[Get up and get ready->Righteous01]]. This links to the twine node titled Righteous01, but it does so with a link that says "Get up and get ready." I added support for these child-name "aliases" in this PR.

[grahamearley]

TODO:

• Put placeholder text that suggests that you copy-paste and don't write json by hand.
• Paste from clipboard button (if possible)

[grahamearley]

Okay I fixed up some things (brace yourself for huge screenshots):

• Default text in JSON text area that suggests you copy and paste into it
• A little JSON validation to make sure you're putting the right format into the textbox. Looks like this when it's invalid:
• A paste from clipboard button! Works perfectly!
• It turns out that we were making the first twine node in the import list be the start node. Well, that's not always how a story is created! Now we explicitly use the startnode field from the JSON to find the right node to activate first.
• Various cleanups