v1.7: apps and plugins

Apps and plugins. unidex now runs apps and plugins that anyone can write, in a few lines of Lua with no tools to install, and there's a store for them.

• Apps (Home > Apps): small apps kept on the SD card. Each runs in a sandbox with its own memory, its own folder and a time limit, and can't reach your WiFi password, keys, notes or other apps' files. An app that breaks shows "App error" with the line, and the device carries on. Hold B on one to offload it (its data stays) or delete it.
• Plugins add to the built-in apps. The first kind adds live badges to Badge, drawn by code each time they're shown (like Today, which always shows the date).
• The Apps page on the website shows what's on your unidex, and installs, updates, offloads and deletes apps over USB. Delete says how much data goes with it first.
• Make your own: read Writing apps for unidex, try it in the browser emulator or on your device, and send it in as a pull request to unidex-apps. It's tried automatically in the emulator and read by a person before it's listed.

Also in this release:

• WiFi: save up to five networks (home, eduroam or work, a phone hotspot). The device joins whichever is in range. The website's Notes page is now called Setup.
• Notes: a recording where nothing is heard is kept as a "Heard nothing" note instead of being deleted; one note that can't be sent no longer holds up the others; and a second recording in the same wake no longer says "Not enough memory".

To update, use Update on https://forrest404.github.io/unidex/ (your settings and files are kept). Every change is in CHANGELOG.md.