ProjectExplorer: Rename Task::addMark to setMark and make it private

Make TaskHub a friend of Task so that it can still use the method. This
should stop anyone from accidentally calling that.

Change-Id: I36974d0ba874f0cac59b7c57ed667ce2e75e1d4c
Reviewed-by: Eike Ziller <eike.ziller@theqtcompany.com>

src/plugins/projectexplorer/task.cpp

@@ -83,10 +83,10 @@ Task Task::buildConfigurationMissingTask()
                 Constants::TASK_CATEGORY_BUILDSYSTEM);
 }
 
-void Task::addMark(TextEditor::TextMark *mark)
+void Task::setMark(TextEditor::TextMark *mark)
 {
+    QTC_ASSERT(mark, return);
     QTC_ASSERT(m_mark.isNull(), return);
-
     m_mark = QSharedPointer<TextEditor::TextMark>(mark);
 }