C++: add code-model settings to choose one by mime-type.

The model-manager now supports multiple code models for semantic
highlighting and code completion, and will choose one based on the
mime-type of the editor.

The settings page is currently disabled. It will get enabled when a
second plug-in lands that has a ModelManagerSupport class.

Change-Id: I10023f52322ed6860397da15dba1c231e80e6517
Reviewed-by: Nikolai Kosjar <nikolai.kosjar@digia.com>