Improve selection tree form widget

[ymarcon]

• Improve selection tree widget (if needed, see this tree component reference).
• Make a script to generate a selection tree schema form from an excel sheet

[ymarcon]

To fix:

• [x] currently when all node siblings are selected, the parent is selected but this behavior does not propagate to the upper level: the parent of the parent does not get selected when all the siblings of the parent are selected
• [x] use the checkbox indeterminate attribute to render that some lower nodes are selected but not all (like in this issue's mockup) otherwise when nodes are collapsed it is difficult to see whether it contains a leaf that is selected, see What's the best way to implement a single tri-state checkbox within angularjs?

[ymarcon]

• [x] allow single selection (string type)
• [ ] selection dialog/popup for both string and array types
• [x] collapse/expand all nodes buttons