Scripting: Add Poll Function & bl_icon to Custom Node Sockets

Title is self-explanatory, custom node sockets don't have a poll function so they show up in the group menu for every custom node tree type, and custom sockets don't have an icon in the group menu because there is no bl_icon property that can be used for that.

Also the addition of the poll function would mean that it can show up in the group menus for built-in node trees as well.