Change demo to add by default, replace if holding shift
Reverse iteration order, so addPack effectively prepends (vs appends)

This allows new packs to be added taking precedence over the existing
packs, instead of vice versa. Makes more sense since the new artwork
shows up immediately, whilst still falling back to the old artwork.
Refresh (invalid blob URLs) after loading packs

Fixes new textures showing up when adding new (prepanded) packs.