Ability to auto-load choosen object in "Select Object to Add" dialog

Steps to reproduce:
1) Open two PDB files

2) Unload one of them

3) In the remaining one, click

button on 3D View toolbar
Expected state: opened "Add" dialog with objects tree
Unloaded objects are disabled (see screenshot), you have to load them manually from context menu, before selecting

Feature: ability to select and auto-load unloaded objects.