Fix Quick Open async-related bugs & performance problems:
- #1855 (Typing quickly in Go to Definition goes to wrong location on
  Enter) -- moved Enter/Esc handling to keyup and added async delay to
  parallel how letter keys are handled; register keyup handler earlier so
  it can block Enter from being processed by Smart Autocomplete (which we
  did before by killing the UI before the keyup could arrive)
- Remaining bit of #1627 (Quick Open doesn't scroll based on prepopulated
  text if opened via menu) -- moved code from keyup to new
  _handleShowResults() method
- Slow performance when typing qucikly in big files like codemirror.js --
  added staleness check in _filterCallback()

Also, some code cleanup:
- move code that didn't need to be in keyup into _filterCallback()
- remove keydown handler now that it's unneeded
- add more docs on event ordering, etc.
Merge pull request #2548 from adobe/pflynn/quickopen-async-issues

Fix Quick Open async-related bugs & performance problems