GTK4

Warning: this branch must be squashed into a single commit before being merged

(I've separated the commits to "ease" the review, but the intermediary commits don't even build until the last quarter of the branch. And it ain't gonna be an easy review 🙁 - sorry)

Switch to GTK4 and libadwaita. Many libhandy widgets are 1-by-1 replacable by their libadwaita correspondents. Some small adjustments needed to be done to ensure margins and paddings are still correct.

I'm pretty sure I got the code style wrong, please forgive these mistakes. I'd like push one last style fixes commit before landing it.

Here's how it looks:

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To Do:

• When opening app details, the title gets selected
• There are a few warnings in the debug log. They seem harmless, but it'll be even better to not have them.