Resolve "Make NautilusWindowSlot non-derivable"
This MR request has 2 purposes:
- Closing #1550 (closed).
- Cherry-picking a commit from Ernestas's early GTK4 port, because
2.1. it applies cleanly between the other 2 commits;
2.2. it removes some direct access to GdkEvent structs, preparing for GTK4.
Edited by António Fernandes