Toolchain: Add option to make toolchain readonly

Do not just disable the toolchain when it is read-only. Add a method to
make it read-only instead.

Task-number: QTCREATORBUG-4211

src/plugins/projectexplorer/toolchainconfigwidget.h

@@ -68,6 +68,8 @@ public:
 
     virtual bool isDirty() const = 0;
 
+    virtual void makeReadOnly();
+
 signals:
     void dirty(ProjectExplorer::ToolChain *);