Refresh on return-to-foreground

I assume `AppKit` fires some kind of `NSNotification` when a window gets foregrounded -- d'you think it could be an enable-able feature to have Postico refresh the open table on foregrounding? Doesn't have to be enabled by default but could be enabled via preferences perhaps.

Start by investigating how AppKit reports a window returning to the foreground, including the relevant NSNotification, and how Postico currently handles the open table. Define the preference behavior and verify that enabling it refreshes the open table on foregrounding without making it default.

Written by the indexing model from the issue text.