Save & restore scroll pos & cursor/selection of working set files across
sessions, and of ALL files within a single session.

Simplify how DocumentManager saves working set state via a new
"beforeAppClose event". There's one ugly bit, a hack in EditorManager.
_restoreEditorViewState() to get around some CM API fragility.

Also save & restore scroll pos more precisely in Quick Open when user
closes it via Esc. Remove some vestiges of the very early, incomplete &
disabled code that dynamically previewed documents as you type. Plus small
cleanup in ModalBar for clarity.
Merge remote-tracking branch 'origin/master' into pflynn/preserve-scroll-pos

* origin/master: (236 commits)
  Update README.md
  Update README.md
  Update README.md
  Clarified comment
  Change string literals for resizeEditor() arg to constants
  Cleaned up refresh code and made it so we always refresh after showing the editor. Addresses some cases in #2782.
  Update CodeMirror SHA. For real this time.
  Update CodeMirror SHA
  Update CodeMirror SHA to brackets-sprint20 branch, which contains a temporary cherry-pick of a fix from upstream and avoids other fixes we don't want yet.
  Update CONTRIBUTING.md
  fix exception when opening empty folder and file open
  Remove extra divider from help menu.
  Reverting to old screenshot since the styling of the new screenshot doesn't match the (reverted) background.
  Updated de to match en Getting Started CSS
  dimgray background
  Turn off newline creation when autoclosing tags
  Set text cursor.
  Update comments.
  Only skip refresh if the width hasn't changed
  prettied it up a bit
  ...

Conflicts:
	src/editor/EditorManager.js
	src/search/QuickOpen.js
Remove hacky workaround that is unneeded in CMv3
Code review: use more standard triggerHandler() instead of trigger(); fix Closure
docs typo.
Merge pull request #2898 from adobe/pflynn/preserve-scroll-pos

Remember selection/cursor and scroll position per file