Improve node editor UI

Several improvements and new features :

• overall code improvements (better click events handling, clearer build process for the graphical items layout, better path for view edges, etc),
• it is now possible to connect nodes using drag and drop from/to ports (see video below),
• there is a visual feedback when the node has still not be applied at least once (green background color, same as in dirty state)
• New interactions :
  • delete pipeline node using middle click on the node label,
  • delete all inputs of a port with middle click on its label,
  • shift + click on the pipeline node label can also remove it from the current selection

output