CppEditor: Clean up header error indicator

* Show/hide the error indicator button instead of enabling/disabling it.
* Use "Minimize" instead of "Do Not Show Again" in the info bar button
  and use a custom setting to save this. The current info bar API does
  not signal addition/removal of global suppression ids which would be
  needed to update all editor widgets properly. We are the only client
  and it feels wrong to add this API there at the moment.
* Remove not needed code anymore.

Change-Id: I2bb872522b7410434f060cc359a3b62dfed0af4d
Reviewed-by: David Schulz <david.schulz@qt.io>

src/plugins/cpptools/cpptoolssettings.h

@@ -63,8 +63,12 @@ public:
     bool sortedEditorDocumentOutline() const;
     void setSortedEditorDocumentOutline(bool sorted);
 
+    bool showHeaderErrorInfoBar() const;
+    void setShowHeaderErrorInfoBar(bool show);
+
 signals:
     void editorDocumentOutlineSortingChanged(bool isSorted);
+    void showHeaderErrorInfoBarChanged(bool isShown);
 
 private:
     Internal::CppToolsSettingsPrivate *d;