Dartvel 0.13.2

Breaking (alpha): @DVModel(generatePublicPages: ...) is now generatePages. A generated model page was never necessarily public: the model's view policy decides who may open it, so a model's pages can be private and auth-guarded. dartvel migrate-code rewrites old code.

• Every page's scrollbar stays on screen and draggable whatever theme the app brings. Before, only apps on Dartvel's default theme got it; an app's own scrollbar colours are kept.