Improve user interface for axis linking

Several adjustments to UI as discussed with Håkon and Alireza.

* add link/unlink command to right-click menu of axis
* select curve in project tree when clicking on legend item
* make sure internal legend is always visible when toggling plot
* fix issue when using sub plot as data source for axis range

Fwk/AppFwk/cafUserInterface/cafUiAppearanceSettings.h

@@ -47,6 +47,9 @@ public:
     QString autoValueEditorColor() const;
     void    setAutoValueEditorColor( const QString& colorName );
 
+    static QString globaleValueButtonText();
+    static QString localValueButtonText();
+
 private:
     QString m_autoValueEditorColor;
 };