Break up massive View module

Before: src/View.elm is one 1676-line module containing all view-related code.

After: src/View/ is a directory with 10 modules organized according to "part" of the view, with no module > ~0.5 kLoC.

It's a large diff but I mostly just moved functions into new modules and adjusted imports accordingly.

This work also felt germane:

• Removed lots of superfluous uses of the word 'view' in function names, because these functions are all in the View.whatever namespace already
• Moved a few helper functions into View.Helpers
• Removed a bit of dead code in View.elm