CppTools: CMM::completionAssistProvider takes a mimeType

No need to pass more than needed.

Change-Id: I047cc44f17342999e0809d022df35ba9a4a8dfb8
Reviewed-by: Erik Verbruggen <erik.verbruggen@digia.com>
CppTools: CppHighlightingSupport takes a BaseTextDocument

...instead an ITextEditor.

The ITextEditor was only used to access the file path and QTextDocument
via the BaseTextEditorWidget.

Change-Id: Idef2b8162b992fe3718e16e2a046e2ff214846f2
Reviewed-by: Erik Verbruggen <erik.verbruggen@digia.com>
CppTools: Start to use editor document

This is only a small refactoring step towards a editor-free
CppEditorSupport.

Left are m_editor references that need to be addressed.

No functional change.

Change-Id: I9fc7b0abfe88f5b9a6ce3f57a5c4a922ae953ddf
Reviewed-by: Erik Verbruggen <erik.verbruggen@digia.com>