ClangTools: Add diagnostic mark class

These marks can now be disabled.
This greys out the annotation color and the icon of the mark.

Change-Id: I5af4591db4baaaef55c986252f77d5d977427b56
Reviewed-by: Christian Stenger <christian.stenger@qt.io>

src/plugins/clangtools/clangtoolsdiagnostic.h

@@ -53,6 +53,7 @@ class Diagnostic
 {
 public:
     bool isValid() const;
+    QIcon icon() const;
 
     QString name;
     QString description;