options: enable separate screen focus by default

When the user closes the active window, with separate screen focus disabled, a window on the other screen might get activated, which also switches the active screen to the other one. As this is quite unintuitive, and in my testing having separate screen focus enabled didn't have any other unintuitive side effects, this commit enables separate screen focus by default.

Afaict there's no need for a kconf update script, as the config only contains an entry if the option is already set to true