Improve click simulation

Improve click simulation

- Don't leak 'command' events to web pages. They're only needed for XUL.
- Don't generate 'mousedown' and 'mouseup' when simlating clicks for browser UI
  elements. They seem to sometimes trigger some buttons' actions twice. It also
  caused background tabs to be selected before closed when clicking on their
  close button, which caused the selected tab to change. However, an exception
  had to be made for tabs. 'mousedown' seems to be the only relevant event
  there.