Merge demo and examples into a single app

[mhsmith]

We talked about this before, but I can't find any existing issues.

It would be useful to have a single app that lets you interactively use as many of Toga's features as possible. Not only would this be useful for our own internal testing, it would also be a good demonstration for programmers who are considering using us.

We could also release it on the App Store and Google Play Store, as Kivy has done, though their Android app hasn't been updated since 2016 and currently refuses to install on my newest phone.

[freakboy3742]

Some additional notes:

1. We have toga-demo, but it hasn't been updated for a long time, and it uses widgets that aren't currently available on iOS/Android. This should be the starting point
2. We also have examples/examples_overview; that's a 'meta-launcher' example - it might be worth preserving the ability to launch a single demo from the command line, rather than requiring a (e.g., toga-demo slider would show the slider as the top-level widget, rather than the full widget "menu");
3. We need to resolve the problem of "top level navigation". We've got enough widgets that we can't put them all on one page; we effectively need a landing page that gives the user a choice, which takes them to specific sub-pages that demonstrates features of individual widgets by rendering multiple versions of the sample widget. Each of the the existing example widgets would effectively be a page on this "top level" menu (although the examples themselves probably need to be cleaned up and simplified).
4. In addition to the "upload to App Stores" goal, this app should aim to be sufficiently rich that it can be a bug demonstrator. We should be able largely enforce a requirement that any reported bug in a widget should require a diff against the demo app making a change that causes the bug to occur.
5. When auditing the examples for inclusion in the new toga-demo, we should ensure that the X demo only demonstrates X. At present, the Button example also demonstrates widget positioning, button colors, and more. There should be a panel to demonstrate widget colors, a panel to demonstrate fonts, a panel to demonstrate widget positioning, etc.