`storagePromptState`; used by `ext-storage.js`
- Fix (regression): Ensure storage dialog will not be blocked because of
canvas updating done for sake of centering background
- Fix (extensions): Ensure `langReady` changes are available by time prefs
dialog is closed and that its changes have occurred by time extensions
have first loaded (`setLang` now returns a Promise rather than `undefined`
as it waits for extension's `langReady` to resolve); this is also useful
with `ext-storage.js` so we know that `extensions_loaded` (which
conditionally updates the canvas based on `storagePromptState`) has seen
`langReady` and the storage extension hasn't set a `storagePromptState`
of "waiting"
d13f99bb5e