Add tree nodes dynamically

New features:
* By clicking on a node in the D3 tree chart:
  - on a short press, a new node is added, stemming from the clicked node (if the node type allows it)
  - on a long press, the clicked node is deleted and the weights of the siblings are reset (after the user presses ok on a confirmation dialog, and only if the node type allows it). In case a theme is removed, also all it's indicators are removed at the same time.
* A tooltip displays the node's description (to be improved)

Open issues:
* Notification messages are too much while managing the weighting tree
* We should avoid calculating at all, in case no indicators (risk or socioeconomic) are in the tree
Defining ``ri_node, svi_node = project_definition['children']`` would be clearer than using the comments