Prevent multiple instances of QWatson from reading/writing concurrently on the same database files

[jnsebgosselin]

Currently, it is possible to open multiple instances of QWatson that can read/write concurrently on the same database.

This is a huge potential source of crash, bugs, and conflicts. In order to prevent that, we would have to implement a system that locks the database files from being written when already opened in an instance of QWatson.