CppTools: Cancel parsing if editor is closed

The m_parserFuture.cancel() in ~BuiltinEditorDocumentProcessor() did not
cancel anything. Thus, closing a document while the parser was running
led to a blocking UI thread.

Now it cancels at the next include directive it encounters.

Change-Id: I092fddbbd747e0bc95265b6e9b4fcc26b3f76cb3
Reviewed-by: Friedemann Kleint <Friedemann.Kleint@qt.io>
Reviewed-by: Tim Jenssen <tim.jenssen@qt.io>

src/plugins/cpptools/cppsourceprocessor.h

@@ -59,6 +59,9 @@ public:
     CppSourceProcessor(const CPlusPlus::Snapshot &snapshot, DocumentCallback documentFinished);
     ~CppSourceProcessor();
 
+    using CancelChecker = std::function<bool()>;
+    void setCancelChecker(const CancelChecker &cancelChecker);
+
     void setWorkingCopy(const CppTools::WorkingCopy &workingCopy);
     void setHeaderPaths(const ProjectPartHeaderPaths &headerPaths);
     void setLanguageFeatures(CPlusPlus::LanguageFeatures languageFeatures);