core: Never automatically switch focus in strict focus mode

This MR follows up discussion from !3050 (closed). The first commit here removes window_is_terminal and its associated helper var. The second commit then removes the focus_new_windows preference entirely; with window_is_terminal gone there is no functional difference between "smart" and "strict" focus mode. As observed in #2106 and noted in the first commit's message, there was likely no behaviour change to begin with, at least on Wayland in most current circumstances. The second commit aims to address comments in #2106 by amending strict focus mode such that windows will not automatically take focus unless they are an ancestor to the transient.