Draft: Accessibility

A very, very simple implementation of accessibility keybindings in Mutter.

So far, I have not been able to track down a few issues to even get all the features necessary to MR with mutter's master branch.

Here's a TODO:

• Get basic version working with existing functionality from keybindings.c almost exactly the same. Be able to define keys via schema files/via gsettings.
• Fix nullptr issue with flags variable.
• Allow capslock as a modifier (if that is not done already)
• Asses whether to re-send the entire set of keybindings whenever a change is needed, vs. using groups of bindings and enabling and disabling them in bulk. (Generally, screen readers use the same keys, but enable/disable certain bindings based on various context clues and user preferences).
• Change permission structure as to need to be allowed for a specific client via a settings module (probably involves more code/schema changes, maybe even in other projects?)
• Plumb DBus endpoints (xdg-desktop-portals-gnome)