New implementation of auto save

As discussed in #58.

Rather than saving upon a change and 1s of inactivity (which is what happens now), this _only_ saves a file

* on editor blur (including split switch)
* on switching files (goto UI etc.)
* on window close

This hopefully should address the issues listed in #58 (e.g. tools using file watch functionality) largely without having to drop auto save.

It feels a bit scary to make this change, though, so please look at the code closely and test thoroughly.