Add title to tree element

[JulienChavee]

When you have functions or variables that have too long names this one isn't showed completely.

A rapid fix for me was to update the lib/tree-view.coffee file:
@li class: 'list-nested-item list-selectable-item', title: label, =>
and
@li class: 'list-item list-selectable-item', title: label, =>

but has I'm not expert with coffee script I'm not sure this solution is good/enough for all cases.