TreeView keyboard navigation

[Harry Mallon <hjmallon gmail com>]

Hi all,

I posted a change to Baobab (the disk usage analyser) which added support for navigating the treeview where:

* Right arrow unfolds selected item
* Left arrow folds the selected item
* Left arrow moves cursor to parent if was already folded
* Space bar toggles folding on current item

See below: https://gitlab.gnome.org/GNOME/baobab/merge_requests/7

It was suggested there that I provide a merge request upstream. Is there anything I am missing which is 
already in TreeView? Is there previous discussion of these shortcuts? If not would a merge request be 
considered for them?

Thanks,
Harry Mallon