Improve the way the Files daemon works

[Zisu Andrei]

As discussed on Slack, an update might the API compatibility between the daemon and the client, as follows:

Run files, which runs the daemon
Update Files
Close the files window, but the daemon runs in the background
Run files again

The new version of the Files application will run, but the daemon is still the old one, which will prevent the application from running.

In Mail, this is handled by the daemon and application being part of the same process: the application is the daemon. It starts in the background when the session starts and when you open the application, it sends a (I can't remember if a signal or a dbus request) to the daemon, which launches an application window. When you close it, it only closes the application, not the whole process.