Fix editor tests and focus/blur issues

* Fix the editor unit tests that were causing errors (but not test
  failures) because the test was not waiting for the promise from
  `Editor#edit` to resolve before deleting the grid.
* Move closured variables to the instance to resolve issues with state carrying
  over between tests
* Blur any active editOn editor before activating another one to avoid errors
editor: Fix other focus/blur-related errors

* Clear focus timeout in destroy to avoid blur-on-destroy errors
* Avoid causing errors in editOn editor blur logic
Don't show the editor until the blur on the previous one is complete

This involves using a timer and showing the editor in the next turn for IE.
CHANGES.md: Note editor focus/blur fixes
editor: Rename _focusedCell to _focusedEditorCell to disambiguate
Fix #841: ColumnHider: Avoid IE10 header cell bugs with ColumnResizer

This is resolved by applying the IE8 workaround (which was done for a
slightly different reason) to IE10 as well.
CHANGES.md: Note IE10 ColumnHider/ColumnResizer fix
Fix #902: Keyboard: Don't fire events for automatic refocuses
CHANGES.md: Note Keyboard event behavior change for automatic refocuses
Fix #917: Emit dgrid-cellfocusout from correct element
Keyboard: Add unit test to verify dgrid-cellfocusout event target
CHANGES.md: Note Keyboard dgrid-cellfocusout event target fix