Remove ATK

To build a better world sometimes means having to tear the old one down.
        -- Alexander Pierce, "Captain America: The Winter Soldier"

ATK served us well for nearly 20 years, but the world has changed, and
GTK has changed with it. Now ATK is mostly a hindrance towards improving
the accessibility stack:

 - it maps to a very specific implementation, AT-SPI, which is Linux and
   Unix specific
 - it requires implementing the same functionality in three different
   layers of the stack: AT-SPI, ATK, and GTK
 - only GTK uses it; every other Linux and Unix toolkit and application
   talks to AT-SPI directly, including assistive technologies

Sadly, we cannot incrementally port GTK to a new accessibility stack;
since ATK insulates us entirely from the underlying implementation, we
cannot replace it piecemeal. Instead, we're going to remove everything
and then incrementally build on a clean slate:

 - add an "accessible" interface, implemented by GTK objects directly,
   which describe the accessible role and state changes for every UI
   element
 - add an "assistive technology context" to proxy a native accessibility
   API, and assign it to every widget
 - implement the AT context depending on the platform

For more information, see: https://gitlab.gnome.org/GNOME/gtk/-/issues/2833

README.md

@@ -56,7 +56,6 @@ building for:
   - [Pango](https://download.gnome.org/sources/pango)
   - [Epoxy](https://github.com/anholt/libepoxy)
   - [Graphene](https://github.com/ebassi/graphene)
-  - [ATK](https://download.gnome.org/sources/atk)
   - [Xkb-common](https://github.com/xkbcommon/libxkbcommon)
 
 If you are building the X11 backend, you will also need:
@@ -70,7 +69,6 @@ If you are building the X11 backend, you will also need:
     - xcursor
     - xdamage
     - xcomposite
-  - [atk-bridge-2.0](https://download.gnome.org/sources/at-spi2-atk)
 
 If you are building the Wayland backend, you will also need: