Add VSPackage to Add New Context Menu

Question

I try to add my VSPackage to the Add New Menu in the Solution Explorer Context Menu Item, but I can't find a way how to do this. Currently I only can add it to the Project Context Menu with the key IDM_VS_CTXT_SOLNNODE. But where can I find all keys that are possible? Because I can't find IDM_VS_CTXT_SOLNNODE in no listing from Microsoft.