Toga

Finally - 0.3.0 in a non-prerelease form! It's been over 5 years since we started the 0.3.0 refactor - and we probably should have cut a release a lot sooner - but finally it's here! The full list of changes and bug fixes are too numerous to mention; but the banner features of this release are:

* Widgets now use a three-layered (Interface/Implementation/Native) structure.
* A GUI testing framework was added.
* A simplified "Pack" layout algorithm was added.
* Added a web backend.

Preview release 0.3.0 dev39.

Includes:
* Moves to a CI-based release process
* Numerous fixes for Android issues stemming from the move to Chaquopy in Briefcase 0.3.10.
* Restores the Toga-web backend
* Adds the ability to create an image from data.