FRAMESET frames should not be registered.

Somewhere along the road, this got goofed up.  This reverts to the
intended behaviour.
Remove the custom scrollbar styling

IMO the default styling looks a lot better, at least on OSX
Allow more exclusion rules before showing a scrollbar
Hide the smooth scroll option from the settings UI

See #1299 for context.
Merge remote-tracking branch 'upstream/master' into post-1.46