Allow input capture & emulation being disabled (#158)

* Input capture and emulation can now be disabled and will prompt the user to enable again.

* Improved error handling to deliver more useful error messages

input-emulation/src/dummy.rs

@@ -26,4 +26,7 @@ impl InputEmulation for DummyEmulation {
     }
     async fn create(&mut self, _: EmulationHandle) {}
     async fn destroy(&mut self, _: EmulationHandle) {}
+    async fn terminate(&mut self) {
+        /* nothing to do */
+    }
 }