Maya - Dockable windows boilerplate

So, here's an update boilerplate. Details in comments.

https://gist.github.com/Nixellion/f28051f1b53fee8b11a2fcd36ca244c3

This one is working in Maya 2016, 2017, 2018, should work in previous versions down to 2014. Still going to test it thoroughly in all versions.

Turns out I was just not calling .show() properly, and MayaQWidgetDockableMixin works properly in older and newer versions of Maya, my bad. I will likely be updating this gist as time goes.