This is a major refactor which updates GTK, GLib and GTK Layer Shell to their latest versions.
GLib channels, previously used for receiving events on the GLib Main Context thread have been deprecated and a new method for running Futures on the main thread has been added instead. This commit also replaces all the deprecated code with this.
As part of the above, a bug was uncovered related to creating the GLib main context inside the Tokio runtime. Spawning of Tokio tasks has been refactored to fix this.
This is a first pass towards trying to structure things a bit better, with data generally encapsulated under a single hierarchical tree, rather than lots of globals all over the place. Lots of work is still required.
The plan is that with this and some more work, #291 should become a lot easier to sort.
