Thumbnails, UI refactoring, World Space, Hardware, Posts and Flippers

[freezy]

This PR goes along with freezy/VisualPinball.Engine#447.

• Add a set of hardware assets (nuts, screws, bolts), mostly for 6-32, 8-32 and 10-32 gauge sizes. Also includes multiple types of washers and spacers
• Add plastic and metal posts from vbousquet/pinball-parts.
• Add some flippers from vbousquet/pinball-parts, plus a few remodeled ones (namely the Bally 2-1/4" shafts).
• Add proper thumbnail support. This adds a Unity project with scripts that auto-generate thumbnails for each asset, with DXR enabled.
• Adapt scaling of all assets to be in world space.
• There were a few models added that didn't make it into the library because I need to set up the environment first, namely a plunger, a DE coin door.