Show endPoint Solid position/mass/velocity info in field editor for Slot PROTO

[stefaniapedrazzi]

We often use Slot PROTO to better handle robot extensions, but if a PROTO has Slot type, the position/mass/velocity tabs are not visible when selecting the node in the scene tree. However, if the endPoint node is a Solid it could be very useful to be able to display these properties in the scene without having to convert the PROTO node to base nodes.