ContentView: construct shellModel inside

* This is the first in a series of commits that removes the
"combined model" {acornModel: acornModel, shellModel: shellModel}
approach, in favor for keeping only acornModels in the main classes
(Player, PlayerView) and only constructing ShellModels when necessary.
Registry: moduleWithId
PlayerView: combined model -> only acorn

See 87af71af1bc58b94a68d8eca1b9865bf444acf99
Player: only track acorn model

See 87af71af1bc58b94a68d8eca1b9865bf444acf99