C++: Base parsing on editor document instead of widget

This mainly takes CppEditorSupport apart.

* Parsing is now invoked by CPPEditorDocument itself by listening to
  QTextDocument::contentsChanged().

* Upon construction and destruction CPPEditorDocument creates and
  deletes an EditorDocumentHandle for (un)registration in the model
  manager. This handle provides everything to generate the working copy
  and to access the editor document processor.

* A CPPEditorDocument owns a BaseEditorDocumentProcessor instance that
  controls parsing, semantic info recalculation and the semantic
  highlighting for the document. This is more or less what is left from
  CppEditorSupport and can be considered as the backend of a
  CPPEditorDocument. CPPEditorDocument itself is quite small.

    * BuiltinEditorDocumentProcessor and ClangEditorDocumentProcessor
      derive from BaseEditorDocumentProcessor and implement the gaps.

    * Since the semantic info calculation was bound to the widget, it
      also calculated the local uses, which depend on the cursor
      position. This calculation got moved into the extracted class
      UseSeletionsUpdater in the cppeditor plugin, which is run once the
      cursor position changes or the semantic info document is updated.

    * Some more logic got extracted:
	- SemanticInfoUpdater (logic was in CppEditorSupport)
	- SemanticHighlighter (logic was in CppEditorSupport)

    * The *Parser and *Processor classes can be easily accessed by the
      static function get().

* CppHighlightingSupport is gone since it turned out to be useless.

* The editor dependency in CompletionAssistProviders is gone since we
  actually only need the file path now.

Change-Id: I49d3a7bd138c5ed9620123e34480772535156508
Reviewed-by: Orgad Shaneh <orgads@gmail.com>
Reviewed-by: Erik Verbruggen <erik.verbruggen@digia.com>