Move common Node/Annotation/MetaItem code into ve.dm.Model

ve.dm.Model is now the common base class for these three. ve.dm.Node
inherited from ve.Node before, so it now uses it as a mixin instead.
This required changing ve.Node's usage of ve.EventEmitter from
inhertiance to a mixin as well, because inherited methods apparently
don't get mixed in correctly.

* Change annotation terminology from linmodAnnotation to element for
  consistency with Node, MetaItem and Model
* Reimplement getClonedElement() in Node for .internal treatment

Change-Id: Ifd3922af23557c0b0f8984d36b31c8a1e2ec497e