CppEditor: Show info bar for project-less file

While at it, centralize the code for handling the info bars and actions.

Change-Id: Ied62e3db268aaff3ef87e246bfe2b50399341be3
Reviewed-by: Leena Miettinen <riitta-leena.miettinen@qt.io>
Reviewed-by: David Schulz <david.schulz@qt.io>

src/plugins/cpptools/cpptoolssettings.h

@@ -66,9 +66,13 @@ public:
     bool showHeaderErrorInfoBar() const;
     void setShowHeaderErrorInfoBar(bool show);
 
+    bool showNoProjectInfoBar() const;
+    void setShowNoProjectInfoBar(bool show);
+
 signals:
     void editorDocumentOutlineSortingChanged(bool isSorted);
     void showHeaderErrorInfoBarChanged(bool isShown);
+    void showNoProjectInfoBarChanged(bool isShown);
 
 private:
     Internal::CppToolsSettingsPrivate *d;