Promises make asynchronous operations easier to manage, in particular
when used through the async/await syntax that allows for asynchronous
code to closely resemble synchronous one.
gjs has included a Gio._promisify() helper for a while now, which
monkey-patches methods that follow GIO's async pattern to return a
Promise when called without a callback argument.
Use that to get rid of all those GAsyncReadyCallbacks!
https://gitlab.gnome.org/GNOME/gnome-shell/-/merge_requests/1126
The Extensions app code is now independent enough from the rest of the code
base to move it to its own subprojects, like we did for the extensions-tool.
This allows for stand-alone builds of the app, which we are about to use
for distributing it as flatpak.
https://gitlab.gnome.org/GNOME/gnome-shell/-/merge_requests/1133
