QmlJS editor: Simplify document/semInfo updating significantly.

* Instead of having SemanticInfoUpdater reparse documents itself, it now
  relies on the ModelManager doing that.
* SemanticInfoUpdater now takes a doc and snapshot to generate the
  convenience Context / ScopeChain. Could be converted into a future
  to avoid having a (99% idle) thread per editor.
* Renamed several functions in QmlJSTextEditorWidget to better indicate
  their behavior.

Change-Id: I8af6ccab099130fa7fa227e44864561ca2c3f9e0
Reviewed-by: Leandro Melo <leandro.melo@nokia.com>