Give warning when ert config file has been modified after gui was opened

**Is your feature request related to a problem? Please describe.**
Very many inexperienced users changes their config after launching the ert gui, thinking their changes will be active.

Potentially leading them to simulate an entire ensemble before identifying the mistake or leaving them wondering why they get the same error over and over.

I would admit as an experienced user I have also fallen into this trap and forgotten to do the restart :sweat_smile:

**Describe the solution you'd like**
It would be nice to as a minimum get a pop-up warning that the config has been modified since the gui was launched. Telling the user to restart the gui to reload the new configuration.

**Describe alternatives you've considered**
The even better solution would be to get a button with a "reload ert" option together with the warning.

No files, tests, or entry points are named. Start by tracing how the GUI loads the ERT configuration at launch and how configuration changes could be detected; done means users receive a clear warning and, if the reload option is chosen, the new configuration is applied safely.

Written by the indexing model from the issue text.