Tests: Add a test plugin to monitor application state

This is a follow-up to !99 (merged).

The idea is to get rid of external and platform-dependent tools like xdotool, in favor of an internal tool that listens directly to the application signals.

Besides the gain in simplicity (once the plugin skeleton is added) and robustness, this opens the way to the use of other APIs for the simulation of user interactions, such as ATK mentioned in !99 (comment 31976).