Don't clear selection list before file save

[alekamca]

There is a callback registered here: https://github.com/AnimalLogic/AL_USDMaya/blob/develop/lib/AL_USDMaya/AL/usdmaya/Global.cpp#L140
which clears the selection list before file save. I see from the comment why this is necessary, but this affects all scenes after the AL usdmaya plugin has been loaded, even scenes which do not have any AL nodes. I believe there is a much more surgical approach to this that could be applied (e.g., only deselecting what is necessary, or not allowing transient nodes to be stored in the Maya file via MPxNode::setDoNotWrite).

FYI: our current use case does not create transient nodes, so I'm just disabling the callback in our local version, but this is not a long term solution.

[wnxntn]

As mentioned above, the selection is cleared in preFileSave()

This deviates from the expected Maya behaviour and breaks selection-based publishers

We are planning a solution where we store the selection list in preFileSave() (ignoring AL_USD proxies as they will never be part of selection-based publishes),still clear the selection list, and subsequently restore the selection in postFileSave().