Simplify async code

- Update Cargo.lock
- Use new gtk-core API
- Remove unneeded async
- Remove unneeded clones

Part-of: <https://gitlab.gnome.org/GNOME/loupe/-/merge_requests/310>

Cargo.lock

@@ -867,9 +867,9 @@ dependencies = [
 
 [[package]]
 name = "glib"
-version = "0.18.1"
+version = "0.18.3"
 source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "331156127e8166dd815cf8d2db3a5beb492610c716c03ee6db4f2d07092af0a7"
+checksum = "58cf801b6f7829fa76db37449ab67c9c98a2b1bf21076d9113225621e61a0fa6"
 dependencies = [
  "bitflags 2.4.0",
  "futures-channel",