Increase delay to tab previews on hover

[bogas04]

I personally find the tab previews to be very abrupt which becomes annoying sometimes.
Ideally we should allow the timings to be configurable, and probably we can choose a default that everyone is happy with, via some telemetry or voting.

[psimyn]

Thanks! Definitely in favour of removing the 0 (prevents flicker when mousing up to url bar then back down). On the fence about going as high as 750ms, but open to increasing it from current.

Related issues for this:
#142
#720

[bridiver]

The delay should probably be configurable as an advanced setting eventually. I'd actually prefer a slightly shorter delay with no preview mode

[bbondy]

there's some concern that it looks like lag when there's a delay, I'm going to close this in favour of the above 2 issue links and possibly a refactoring of how this works completely.