Show scroll track tickmarks for Find results

One more bonus feature to squeeze in this sprint, if anyone has cycles to review it :-)

This shows yellow tickmarks in the scrollbar track showing the position of each search result (similar to Chrome, WebStorm, etc).  On Mac, where there's no scrollbar visible normally, the tickmarks just float on the right hand side (not sure if this is what those other apps do too).

The tickmarks are disabled for inline editors right now.  Find is somewhat broken in inline editors anyway, so it's probably not a big loss.

The tickmark API isn't intended for reuse elsewhere yet, but I think eventually we should generalize it so it could also be used for linting errors/warnings, ["diff marks"](https://trello.com/c/58tCeGUw/952-continuous-diff-in-editor-gutter), etc.