Handle importing files with ProxyShapes

[alekamca]

Does it make sense to disable executing the post load process when importing in files? I.e., change https://github.com/AnimalLogic/AL_USDMaya/blob/master/lib/AL_USDMaya/AL/usdmaya/nodes/ProxyShape.cpp#L1099

to
if(m_stage && !MFileIO::isOpeningFile() && !MFileIO::isImportingFile() )

[alekamca]

While digging into this more, I've discovered that simply changing that one line isn't sufficient to fully support importing files with ProxyShapes. At the very least, the postFileOpen callback needs to also be called postFileImport. However, in this case, we don't want to mess up any previously existing ProxyShapes by trying to re-initialise them.

I'm working on handling this case now, but it will probably be a bit before I have it working in a general enough way that it's fully tested and ready for a PR.