Click and hover on static plots. Also, fix retina compatibility and make hover delay configurable

Edit: oops, I meant to make this pull request against @jcheng5's branch. But I guess it makes sense here too.

Example app at https://github.com/wch/testapp/tree/master/locator

```
runGitHub('testapp', subdir='locator', 'wch')
```

The code in the original commit actually worked fine with Retina displays - at least, it did for me with this test app.