Fix segfault after deleting monitor

Fix segfault after deleting monitor

The boost signal was sent synchronously, and so made the Qt signal to be posted
in FileMonitor::changed after the boost signal returned, so after the sender was
possibly destroyed.

The solution is to make the boost signal asynchronous using the Qt event loop.

Thanks to Scott Kostyshak for the report and MWE.

(cherry picked from commit 131f4b92bac3ecb75b47c266dfa3d8543bd4d578)