Fix #399: Maya 2019+ hide containers in outliner

[BigRoy]

This implements #399 - simple as that.

• Backwards compatible, it first checks whether the attribute exists.
• Will only set the value on newly created containers/sets.

[BigRoy]

Note, this currently does hide the Main container for Maya 2019 scenes when loading a new instance into an existing scene, but not its members making them "all" visible in the outliner by themselves.

I'll change the code so it only hides the main container on initial creation...

[davidlatwe]

Sorry, currently not able to access Maya 2019 for giving this a try, hopefully there will be some gif :P

[BigRoy]

Here's a little preview.

It shows:

• Containers being hidden.
• Whenever a "set" is in the container, it now directly shows in the outliner. Yay!
• You can hide/unhide them via the Display > Ignore "Hidden in Outliner" option.

[tokejepsen]

Looks good to me :)

[BigRoy]

Will merge this tomorrow if no objections..