Update VE core submodule to master (5f4b740)

New changes:
a047e72 [BREAKING CHANGE] Create ve.dm.Focusable and store focusable flag there

Local changes:
* Add new ve.dm.FocusableNode mixin where required

Change-Id: Ie9ee2de05acfe3c4c62c4864e65bd20397d6a069