Polish NodeEditor example

[x] When node editor presents parent+child connection, make sure that selection of child make the parent deselected.
[x] Introduce PortInfo
[x] Provide an arbitrary amount of output ports.
[x] Introduce a family of nodes: cylinder, box, lattice, layout.
[x] Create the default sample with all items and place it nicely on the graphics scene.
[x] Catch exception on the attempt to connect attempts of the wrong type.
[x] Remove "value" in column name of property editor.
[x] Provide undo/redo.
[x] Provide project save/load.
[x] Create delete item action.
[x] Provide README.

gpospelov / qt-mvvm