Testing closing of several dialogs

Testing opening and closing of:
- git-clone action
- help-about action
- new-project action

Also:
- setup transition-complete event on spark-overlay and listen for it in tests
- refactor access classes into separate file.
- fix several nit issues (copyright and library names)
- checking visibility with `elementFromPoint` instead of `display` / `visiblility`

@devoncarew @ussuri 
Agreed.  This was the discussion where I changed my style rules for `.then` paren. handling: https://github.com/dart-lang/chromedeveditor/pull/2578#discussion-diff-13826713 .  I'm happy to do it either way.  Just want to try to stay consistent.