Make Kivy apps accessible, add `kivy.core.accessibility` abstraction for accessibility providers

[misl6]

In the last few days, I had the chance to chat with one of the https://github.com/AccessKit/accesskit core developers, regarding accessibility in Kivy.

For the last decade, Kivy has evolved quite a lot but never officially supported accessibility features (even if users have succeded in adding such features by themself).

In our vision to make Kivy a solid platform to build Apps for everyone, introducing accessibility features and making Kivy apps accessible by default has become increasingly important.

Kivy 3.0.0 is the right moment to break things and make the appropriate changes to allow accessibility tools to do their job, when they need to.

Even if we will work closely with AccessKit team (as they already have a POC, and offered to write an implementation for their tool in Kivy), Kivy will have an abstraction that will make it possible for other accessibility tools to add their implementation in future.

The plan:

### Tasks
- [ ] https://github.com/kivy/kivy/issues/8597
- [ ] https://github.com/kivy/kivy/issues/8598
- [ ] https://github.com/kivy/kivy/issues/8599
- [ ] https://github.com/kivy/kivy/issues/8600
- [ ] https://github.com/kivy/kivy/issues/8601

[clayote]

Here's the proof of concept thanks to @DataTriny.

[misl6]

@DataTriny and @clayote :

I've just finished the first draft of all the tasks linked to this issue. With no rush, when you have some time, can you please proofread it and share your thoughts?

(mainly https://github.com/kivy/kivy/issues/8599)