misc improvements (use std::move, use const&, etc)

[Quincunx271]

Some other things I noticed:

• ConnectionGraphicsObject.cpp:172

   auto view = static_cast<QGraphicsView*>(event->widget());

  It might be better to make this a dynamic_cast (or Qt has an equivalent IIRC), which would make it an error if it was wrong, rather than silently failing.

• NodeGeometry.cpp
  The _entryWidth member is never set, but it's also never used, as entryBoundingRect() is never called

[paceholder]

Looks good!

Regards,
Dmitry