C++: unify highlighting/code-completion support "factories".

Both semantic highlighting and code-completion go hand-in-hand, so now
the ModelManagerSupport class acts as a "factory" for the model manager.
Depending on the mime-type of the document in the editor, the model
manager will return the appropriate highlighter or code-completion
engine. If none is registered, the built-in fall-back is used.

Change-Id: I3e5dbb0e3b58e077dd5eda9aecb2ce5d448ac0b8
Reviewed-by: Nikolai Kosjar <nikolai.kosjar@digia.com>