Add an option to create a selection area in the designer editor.

This widget already exists 'spinbox', but it is linked with other systems and it is impossible to create our own system with this one.

We should have it on standalone.

(I have created a widget to do this, but it's not necessarily the cleanest).

I created new panels (double side scoll box) but it is impossible to drag another widget into it, I do it with a variable.

I have in child of my user widget a canvas which can have children, I would like to be able to make a bridge between my user widget and his child to allow to add a child in the right place without using a variable which can create conflies in the editor, even if it works.