Add focus and blur events to ce.Surface

Didn't bother to deduplicate blur-focus event pairs that occur when the
focus moves from the documentNode to the pasteTarget or vice versa
(this happens when switching between normal selection and FocusableNode
selection).

Change-Id: If1ccd2fbf11de956b6c2364ae81b9dc20a1bf409
Merge "Add focus and blur events to ce.Surface"