Stop using ve.isMixedIn() to check if a node is focusable

* Moved isNodeFocusable to ve.ce.NodeFactory
* Added isFocusable static property to ve.ce.Node
* Set isFocusable to true on ve.ce.FocusableNode

Change-Id: I3cf666280abdfce55bf9b0710827bb25c40bfd51
Merge "Stop using ve.isMixedIn() to check if a node is focusable"