Towards pure python interactors: mouse/keywoard events

[renoust]

This one is more a request for the long run, to port to python these functions:

• it would be nice to have included in an Observable-type manner mouse and keyboards events
• thus it will be important to be able to read the position of a mouse (and keyboard state) in a tlp.View at any time
• in node-link diagram derived views, it would be wonderful to have additional events such as nodes and edges hovering (or at least to get the z-ordered list of elements below the mouse position)

With these primitives, we could copy the flexibility of more advanced events structures as in javascript, such as hovering, entering/exit, dragging/dropping etc.