Well, I looked one more time. So yes, all my functions are implemented. The plugin and the application use the same header file, so it a yes for the second question.
And, yes too for the libraries.

But, there is hope ! After many tests, I succeed to load my plugin !
Apparently, the problem came from the Item definition. First, I need to declare it as an interface, and next I must remove the Q_OBJECT macro.
However, I can still extend the Item class from QGraphicsWidget, so the only drawback is for the signal (no macro, no signal ...), but I can use another interface to replace this, so it's not very bad.

I will let you know when my plugin will really work : I need to change some things in my program to be able to test the creation of items.