BG - Add Blender dropdown context menu to item lists, for things like copy/paste.

[j-conrad]

Is your feature request related to a problem? Please describe.
The Behavior Graph-related list menus (Behavior Graphs, Variables, Custom Events) lack the ability to copy/paste from one object to another, which is a common place to look for that operation for Blender users. They also lack the operation for duplicating list items, which we currently allow in the Behavior Graph editor itself. But a common Blender design pattern is to have these operations accessible from the lists themselves since the editor isn't always open. Or in the case of our Variables and Custom Events menus, there is simply no way to copy or duplicate those items in some other editor.

Describe the solution you'd like
In Blender, item lists (i.e. Vertex Groups or Shape Keys sections in the Object Data Properties Editor) can have a dropdown menu (referred to as a 'context menu' in some places) added for useful operations. I'm proposing that the Behavior Graph-related lists:

• Behavior Graphs
• Variables
• Custom Events (and Parameters submenu)

have their own dropdown context menus for the following:

• Copy to selected (this copies the item, in this case a Behavior Graph or Variable to any selected objects)
• Duplicate (makes a copy of the item in the list with a new name ( *.001 or whatever suffix makes sense in that context)

Describe alternatives you've considered
The best way to get those items propagated to other objects is to add them all to one object before duplicating it. This is not efficient and is somewhat destructive when iterating.

Additional context
An example of the Vertex Groups context menu: