Add a ThreadPool async executor to State

If we need to use simple async code in a few places, a single executor
may be better than having several threads blocking on async code.

This should probably use the calloop executor, but that's had issues in
cosmic-workspaces, though that may not apply here.

Cargo.toml

@@ -87,6 +87,7 @@ reis = { version = "0.5", features = ["calloop"] }
 clap_lex = "0.7"
 parking_lot = "0.12.5"
 logind-zbus = { version = "5.3.2", optional = true }
+futures-executor = { version = "0.3.31", features = ["thread-pool"] }
 
 [dependencies.id_tree]
 branch = "feature/copy_clone"