Accordion can now spit trees

* Added function tree() which can be used for getting the underlying tree of specified accordion
* This function can also directly click on the tree to save typing
Flatten nav trees (Control, Config)

Thanks to the fact that the accordion can now spit the trees directly, the navigation tree can be flattened
Merge pull request #682 from mfalesni/accordion_tree

Make accordion able to spit the underlying tree