V6 Goal: Decals tuned up

Continuing the discussion from Decals - tune up needed?:

@andy, can you provide more details?

The old decal interface which was based on the Flamingo 2 code was pretty clunky, so one of the V6 rendering priorities was to improve this. So far:

The user is immediately prompted for a bitmap texture.
Placement controls are standard Rhino plane, cylinder and sphere pickers.
Positional editing uses a new decal widget similar to the texture mapping widgets.
The object properties panel is improved with better previews, icon buttons instead of text buttons, an improved texture picker.