MAYA-104776 send a notice that a newly created proxy shape is dirty

[fowlertADSK]

The workflow that shows the problem here:

1. Create a new proxy shape.
2. Right click on the proxy shape and you will see the UFE/USD menu items, e.g. "Add new prim".
3. Create another new proxy shape. Not from file, an empty one.
4. Right click the new proxy shape and you will not see the UFE/USD menu items.

This happens because we maintain a map of proxy shapes <-> USD stages that gets updated only when dirty. The map currently only gets dirtied when the file path attribute on the proxy shape changes, which worked fine until we added a command to create a new empty stage + proxy shape. In this case we were not notifying anyone that there was a new/dirty item so the map was not dirtied and we do not update the popup menu.

[robthebloke]

I think we saw the same issue and kinda hacked around it in a slightly dirtier way. The change looks good, but I'm going to hold off approving until later this evening so I can run this through our integration tests