Delay watcher setup until a target actually gets executed

[fbecart]

At the moment, Žinoma initializes file watchers as it initializes target actors.

The creation of watchers could be delayed until a target is executed for the first time. This would improve the startup time (and the resource usage) of the watch mode.

[fbecart]

This would also fix a bug that occurs when watching directly after a clean: cleaned file are not present from the start, and the watcher currently skips them.