Slow and steady wins the race: lengthen search activation timeout to improve performance

A more realistic default delay avoids spamming the search backends.

I've tested this change with Nautilus' master branch through GNOME Builder, and it works great for me on my end. I considered a 500ms delay but it was a bit too noticeable for things like remote search, so I compromised to 400ms based on my calculations of what would be the minimum value we could get away with. I do not recommend going lower than 400ms, at least not until everything else in the search infrastructure is absolutely blazing fast.

While it does not solve every perf issue, it's an improvement, and it unclutters our view to see the remaining performance problems.

Fixes issue #1731 (closed)