Importing a profile, saving a QR code image or setting an auto-accept directory using Nautilus as the default file manager, which many users use, was causing a hang in Qt's method. Setting Qt to use it's own file manager in this circumstance fixes the issue. Closes#3436, closes#3443.
qTox will automatically import the old history on startup.
This new database code is much more robust.
It is very resilient and will not corrupt or disappear after a crash or
power failure, unlike the old code.
The on-disk database format is also much more compact now.
The database sync option in the advanced settings has been removed,
we know run many database operations asynchronously so performance
should not be a problem anymore, but we always ensure resiliency
in case of abrupt termination, so there is no tradeoff anymore.
