ClangFormat: Refactor indenter to allow ClangFormat unit-tests

We do not build texteditor files in unit-tests so some tricks
were required to make ClangFormatIndenter available.

First simple unit-test proofs it builds and runs.

Change-Id: I81d5ea099bd27fd1c1ed8b5b7877299dcc62a67f
Reviewed-by: Nikolai Kosjar <nikolai.kosjar@qt.io>

src/plugins/cpptools/cppmodelmanager.h

@@ -49,7 +49,6 @@ namespace CPlusPlus { class LookupContext; }
 namespace ProjectExplorer { class Project; }
 namespace TextEditor {
 class BaseHoverHandler;
-class Indenter;
 class TextDocument;
 } // namespace TextEditor