CppEditor: Fix highlighting in new split if file is changed externally

For a new split/editor no CppHighlightingSupport was set since
onMimeTypeChanged was not called.

Now the CppHighlightingSupport is set again in the constructor. This
reverts partly

    commit 033ad887
    C++: handle file type changes for choosing highlighters

Task-number: QTCREATORBUG-9382
Change-Id: Ie79d357d570bec4920366fbd56fb67859be9b37a
Reviewed-by: Erik Verbruggen <erik.verbruggen@digia.com>
Doc: Fix "path" to Import Existing Project

Change-Id: I79ad2360d7cebaa573199efc40bfd92a3c1f006b
Reviewed-by: Leena Miettinen <riitta-leena.miettinen@digia.com>
CppEditor: Add a comment in cppquickfixes.h to keep match() functions cheap

Change-Id: I60bb01df9db9d93752f8b574dfb61d3be35b974e
Reviewed-by: Lorenz Haas <lykurg@gmail.com>
Reviewed-by: Erik Verbruggen <erik.verbruggen@digia.com>