Add-on updates (version 1.0.19)

We worked hard on improving our add-on - again.

Blendkit 2.8 beta

Blendkit (formerly BlenderKit) introduces add-on for Blender 2.8. While there are still many changes in Blender happening, which may cause the add-on will work today and not tomorrow, the add-on should be mostly working fine, and we try to keep the new version updated with the latest Blender builds.