switch to use selenium native click

Too hard and sometimes impossible to emulate a real click using js.
Selenium's version apparently bugs but is at least better than the
previous handmade version: clicking on input focuses it immediately,
clicking on select works (not option though, because
`document.elementFromPoint` doesn't return the option since it's
OS-level stuff, will fix later), and clicking on everything else works
fine.
refactor
remove dangling screenshots from previous recording
refactor. Remove screen shot index field in record.json

Even more hackable and readable now!
new examples
Merge branch 'interval'