Viewer state management refactor

[frasercl]

Review time: medium-large

Refactors state representation out of App in favor of a new component, ViewerStateProvider.

Some background. This repo is, in a sense, concerned primarily with providing a nice UI for manipulating viewer state. By viewer state, I mean any state which is in some sense a setting of volume-viewer - so whether the bounding box is visible is viewer state, but whether the control panel is open is not. Currently, this state is stored and managed like this:

• Viewer state lives in a couple big state objects owned by our top-level UI component (the one we export from this package) which is called App.
• Alongside all of App's other responsibilities - managing image loading, updating the actual viewer, keeping track of basic UI state - we define several setter functions which keep us a bit disciplined about updating viewer state.
• Viewer state and relevant setters are made available to the rest of the UI by drilling each value and function through a chain of props until it gets to the component that needs it.
• There is no good way to access viewer state from outside App, which is a problem for upcoming work on populating URLs with viewer state for sharing.
• There is also no great way to set viewer state from outside App. While App has props for setting viewer state, they only set initial state and do nothing when the component is mounted.

The changes in this PR move us to managing state like this:

• State and setter functions are provided by a new component, ViewerStateProvider.
• This component provides state via a React context, and therefore can be lifted to anywhere in the component tree above App, allowing access to viewer state outside the viewer component.
• This component also accepts props for viewer settings, which are inspected and synced to state on every render.
  • It currently does not accept settings which apply to specific channels... for reasons. We can discuss if this should be added.
• Components may use a new higher-order component, connectToViewerState, to "subscribe" to the props they care about directly, cutting down on prop drilling.

[ShrimpCryptid]

Could you clarify who you wanted to review this PR? (I'm planning to because I'll be working off of these changes, but I saw there are four(!) people listed as requested 😆)

[rugeli]

Could you clarify who you wanted to review this PR? (I'm planning to because I'll be working off of these changes, but I saw there are four(!) people listed as requested 😆)

No worries, I haven't started the review and can opt out!