[BREAKING CHANGE] Create ve.dm.Focusable and store focusable flag there

Eventually all selection logic should be in the model, and as
focusable nodes behave differently, this information needs to
be in the model too.

Change-Id: Ib381510366f8c8ebb7a67af627178490ff6a736e
Merge "[BREAKING CHANGE] Create ve.dm.Focusable and store focusable flag there"