This is an initial implementaiton of GtkAccessibleText which was added
to GTK for 4.14. It attempts to implement things in a very similar
fashion to the previous code for GTK 3 although considerable effort was
made to simplify and improve readability as to how it works.

Currently, this supports reading back what you type and what has changed
on screen. It is not yet 1:1 what the GTK 3 a11y implementation did
because ATK was doing many other things (including proxying keyboard
keys) to the other side of the a11y bus. That appears to improve
readback by screen readers in the form of "backspace" and what character
was deleted.

I expect things to get closer to 1:1 but that work is going to have to
be done inside of GTK itself first and should not require much if
anything here.

A new VteTerminal:enable-a11y feature flag property has been added
because I'm concerned about enabling this by default until the a11y bus
learns to be more lazy. Currently there is no way to "do not...