Feature: Material quick preview

[andyp123]

Although it's currently not possible to view vertex alpha in the viewport, it IS possible to set up an emission shader with vertex alpha transparency to preview vertex color and alpha at the same time.

Painting with alpha is also not working in the latest 2.81 alpha, but having a quick preview would be pretty handy. Implement as a simple toggle so that users can bind a single shortcut to toggle. Perhaps add an option in the add-on prefs that allows the user to use emission, principled, diffuse by default.