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.lock

@@ -833,6 +833,7 @@ dependencies = [
  "cosmic-text",
  "egui",
  "egui_plot",
+ "futures-executor",
  "i18n-embed",
  "i18n-embed-fl",
  "iced_tiny_skia",