build pyside2 and gaffer with it, so we can support maya 2018

[hradec]

we need to build PyQt5 to build Gaffer for Maya 2018.
Lets see if we can get away building PyQt5/Gaffer using Mayas QT5!

Since we already have Qt4/PyQt4, we just need Qt5 to run Gaffer inside Maya 2018 actually!

[hradec]

PySide2 seems easier to build than PyQt5 at this point, so I'm switching to PySide as default for CY2016 and forward.

PipeVFX still builds PyQt4 as pre-QT5 python binding to keep supporting older applications.